Harry was very pleased with himself. Setting up the prank had gone off without a hitch! The Slytherins wouldn’t know what had hit them tomorrow. And he was fairly sure they wouldn’t be able to blame him for it either.

He felt a little bad about inflicting it on the entire house. The rest of the Slytherins were a bonus– really it was just Malfoy who was being as unbearable as usual.

Malfoy had been a total jerk to Ron since the train, and he kept saying things about Harry’s parents too. He had quietened down for about a week after the incident of the Amazing Bouncing Ferret, but now he was back to full strength.

Harry smiled to himself. Hopefully this would subdue him for a while too.

“Who is there?”

Harry froze, checking that his invisibility cloak was fully covering him. It wouldn’t do to let a floating arm or disembodied foot give him away.

“Reveal yourself.”

Looking frantically around, Harry saw Snape emerge from the shadows of a dungeon corridor. He pressed himself into the wall, holding his breath. That had worked before.

“I thought I told you to stay in your common room? I am only giving you one warning.” Snape’s voice was low and threatening. Harry was a little confused, did the professor think he was one of the Slytherins?

 Snape’s dark eyes swept around the corridor, trying to detect any flicker of movement.

“You have precisely three seconds to show yourself. If not, you will be confined to your common room for the weekend.”

Snape didn’t count aloud as Harry was expecting him to. Instead he stood very still, eyes not stopping their constant shifting. It was therefore a shock to Harry when Snape drew his wand and cast a spell with a simple, expansive movement. Instantly, the potions professor turned to look at the spot Harry was standing in.

“Ah, so you are there. Kindly cancel your disillusionment charm, or whatever it is you are using. Then we will- Stop!”

Harry made a break for it. He narrowly avoided the hand which was reaching out towards him, ducking under the arm instead. He practically flew up the few steps which lead to the Entrance Hall and freedom.

Right at the top of the stairs though, a green door was blocking his escape. Harry had never seen one like it. It looked like solid wood but something seemed to be shifting slightly inside it. Like the swirls that appeared if you dropped ink into a glass of water.

He grabbed for the door handle but as hard as he pulled, it wouldn’t budge.

Harry heard unhurried footsteps behind him, before a hand took hold of his shoulder, spinning him around. The cloak slipped a little and Harry realised he was fully visible to the professor.

“Potter?” Snape looked surprised for a moment. Then he smirked, “This is uncharacteristically cowardly of you, Mr Potter. Are you so frightened of earning a detention?”

“I’m not stupid, Sir. Who would go to a detention if they didn’t even get caught?”

“But you very much were caught, Potter.” Snape drawled. “That is what we call it when you are discovered out of bounds after hours.”

Then his face fell and he brought up a finger and thumb to pinch his long nose. He gave an annoyed sigh.

“I did, however believe it was one of my Slytherins who was breaking curfew.”

Harry just shrugged. “There might be one.” He offered, “I don’t mind if you go check. I’ll just head back to my common room and you can get on with your Slytherin duty things.”

Snape fixed him with a glare. “Do not even try, Potter. You will not be able to get through that door. Come along.”

Snape turned to go, but Harry didn’t move.

“Sir, go where?”

“As much as I would like to give you detention all night for disturbing my sleep. The headmaster frowns upon such practices. Since we both have classes tomorrow morning, we are going to bed.”

“But Gryffindor-“

Snape sighed again, “Potter, you cannot go through that door. You do however have a choice about where you would like to sleep. You may sleep in the Slytherin dormitory with your classmates, or you may sleep in the isolation room near my quarters.”

That wasn’t really a choice. If Snape took him into the Common room, he might discover the practical joke which was set up. Even if he didn’t, the Slytherins wouldn’t take too kindly to him having appeared on a camp bed in the middle of the night.

“The isolation room, I guess.”

“Yes, I had rather thought you might choose that.”

Harry didn’t really understand what had happened. Snape seemed to have cast a spell which had told him exactly where Harry was, but which also had the effect of conjuring the strange door which had stopped him leaving the dungeons.

Snape led him down past the Slytherin common room, and down a couple more chambers. Until they reached a dead end wall with a few doors along the edges of the corridor. Snape opened one of them, pulling Harry inside.

It was just a small room, with only enough room to fit two beds. There was a doorway leading to a bathroom as well, but looked like there was hardly any space in there too.

The bed was already made up with white sheets and a small pillow. The whole room was a bit clinical, sort of like the hospital wing.

Snape sighed and Harry looked up at him, nervous.

“Why is it always you who has to cause so much trouble? We have barely been back a fortnight and already you are roaming the corridors at night, causing mayhem.”

Harry tried to appear contrite. Snape just eyed him with distaste.

“I will collect you tomorrow morning at seven o’clock. You will be dressed in the plain robes which are in the bathroom. There should be pyjamas inside the bed.”

Harry nodded, getting out of the way so the professor could leave the room.

“Yes, Sir. Seven it is.”

“And no more wandering, Potter. I will know. Tomorrow I will explain your punishment.”

Snape left without another word, slamming the door after him. Harry considered trying to leave but thought it probably wasn’t worth it. Snape would have to let him leave the dungeons in the morning. It was a Thursday after all, and he had classes. And when he was allowed out, Snape couldn’t make him come back down here.

Harry reconsidered. Technically, Snape was physically capable. It was less likely that the other teachers would let Snape drag him around the place though.

He got changed into the pair of huge, green pyjamas he found under the bed covers. Harry grimaced, it was like wearing Dudley’s old clothes again. One size fits all shouldn’t count if it included people the size of Flint.

As he drifted off to sleep, Harry smiled, thinking of the shock all of the Slytherins were going to get at breakfast.

“Up, get up! Now!”

Harry groaned, rolling over to bury his head in his pillow.

“I’m awake, Aunt Petunia.”

“Indeed?”

The door opened, and Harry was confused for a moment about why he hadn’t heard the locks being undone as he usually did. He peeked up through bleary eyes to see not his aunt, but a very irritated looking Professor Snape standing in the doorway with his arms crossed.

“You were instructed to be prepared by the time I arrived, Potter. Not dreaming about your indulged home life.”

Harry shot up to his feet, swaying for a second as all of the blood rushed from his head. He glared at the professor. What did he know about life at Privet Drive anyway?

Snape’s lip curled as he took in Harry’s scruffy appearance. The boy looked particularly dishevelled this morning. More like his father than ever with his hair sticking up all over the place. There was one blatant difference - James Potter would never have been seen dead in clothes like the ones his son was wearing now.

“Sorry, Sir.” Harry said, stifling a yawn. “My alarm clock is back up by my bed.”

“And you do not know how to cast a Wake-up spell yet? Or how to shrink fit clothes? Have you got some personal vendetta against everyday spells?”

Harry blinked blankly. He didn’t know how to do either of those things actually, but now Snape had suggested them both, it did seem the obvious thing to have done.

“This is why we do not strut around the school at night, Potter. You have three minutes to get ready, then you are coming up to the Great Hall, day robes or not.”

Snape leant against the doorjamb, waiting while Harry sped around the room. Snape had already started walking towards the bathroom door when Harry emerged, fully dressed in the large robes provided, ready to go upstairs.

When Harry made no move to shrink the clothes himself, Snape drew his own wand and let them draw together so that they formed a much thicker, but more compact set.

“Thanks, Professor. That’s really nifty.” Harry was very glad that he wouldn’t have to spend the whole morning in clothes which were 6 sizes too large. Malfoy was going to have a field day as it was.

Harry grabbed his wand and Invisibility cloak from where he had left them on the other bed. He was worried for a moment that Snape might try to take the cloak away from him, but the man just led the way out of the isolation room, towards breakfast. Harry hurried to stuff them both into his pockets.

Snape spoke as they walked.

“Thanks to your infinite stupidity last night, I was forced to detain you here. I had thought that any student in the dungeons after classes had ended were more than likely to be in Slytherin house. Indeed I had been called to the common room earlier in the evening due to students being out after curfew. I expected it to be the same person.”

He half turned back to check that Harry was following him.

“As I expected the student to attempt to flee, I cast a containment spell which would reveal where they were and prevent them from leaving the dungeons. The spell lasts for about four days, so it will end late on Sunday night, in time for you to return to classes on Monday morning as usual. Understand?”

“Not really.” Harry said, “Are you saying I have to stay in the dungeons for all that time?”

Snape grimaced, “You will not be able to leave the dungeons unless you are escorted by myself. You will not be able to walk more than 5 yards away from where I am or you will be forced to follow. You will have to explain to your other professors why you will not be able to attend their classes for the next two days. And you will have to sit at the Slytherin tables during meals.”

“What? Why?” Harry was alarmed, he couldn’t think of any reason why he should have to do that.

Snape merely rolled his eyes at the reaction. “They will not kill you, Potter. And as I have already said, you will be unable to move far enough away for you to sit at your house table while I am at the staff table. The staff have no reason to accept you at their table, and there is absolutely no way I am going to sit with a load of Gryffindor brats.”

“Can’t you just cancel the spell if it’s going to be such a pain to everyone?”

“Unfortunately, that is not possible. The spell only breaks after the allotted time has passed, or if your life would be in danger because of it.”

Harry groaned loudly.

They reached the large green door and Snape put out his hand to pull it open. It swung open as if it had never been locked. Snape took Harry by the shoulder and guided him through into the entrance hall.

“Have you done this before, Professor?” Harry asked, nervously glancing around in case some Gryffindors suddenly appeared.

Snape smirked, “A few times. If you see me sitting with the Slytherins, or if I have a student assisting me in Potions.”

Harry thought he could remember that happening a few times since he had arrived at Hogwarts, but it was by no means a regular thing.

“Of course, each Head of House is entitled to do so. You may notice that Professor Sprout is the most frequent of us to use the spell. She says she uses it as an opportunity to ‘stay connected with her students’,” He scoffed. “Professor McGonagall is not fond of it, however, which is likely why you have not noticed it.”

“But how does it even work? I’m not a Slytherin!”

“Lower your voice!” the Professor snapped. “The spell is actually in common usage. Wizarding parents use it to discipline their children. You’re friends, the Weasley’s, used to be very fond of it. But at Hogwarts, it is only housemasters who have Professor Dumbledore’s permission to use it.”

Harry thought about that, he was pretty sure he had heard Ron complaining about being ‘grounded’ before. The Dursleys had never done anything like that to Dudley, but they didn’t think he ever did anything wrong. When Harry had ever been confined anywhere, it was always to one room; either his cupboard or Dudley’s second bedroom. Other than that he had only really heard about it on TV. It wasn’t a very British thing to do.

When they entered through the doors of the Great Hall, Harry absent-mindedly started towards the Gryffindor table. Snape threw out an arm and steered him towards the front of the Hall instead.

“Not so fast, Potter. You still need to speak to your teachers first.”

The tables he was led past were mostly empty at the moment. It was still early in the day and in a school full of teenagers, most people waited until the last possible moment to get out of bed. There was only the Hufflepuff quiddich team, a handful of Ravenclaws who seemed to be holding some kind of pre-breakfast art club and a couple making eyes at each other over the Gryffindor table.

The staff table on the other hand was almost completely full. The teachers usually did try to get down early so they could get prepared for classes. A few were missing, Hagrid wasn’t there and neither was Professor Sinistra, but they both had to stay up late at night as part of their job.

The Staff watched as Harry and Snape approached. Dumbledore smiled serenely at them, barely distracted from his paper. Moody was looking suspicious of them both, eyes flickering between them, but that was pretty much how he always looked. The rest of the staff were pretty nonplussed apart from Professor Sprout who suddenly grinned broadly and whispered into the ear of McGonagall next to her. McGonagall’s mouth twitched in one corner.

Harry groaned quietly.

“It’s not you they’re laughing at, Potter. Do be quiet.” Snape muttered to the side.

“Professor Snape, you seem to have caught a lion.” Dumbledore chuckled. “Or put one on a lead, perhaps.”

“I thought he was one of my older Slytherins, disillusioned” Snape said. “I had forgotten about Potter and his blasted invisibility cloak. At the time I was not aware it was him I would be stuck with. Potter, say your piece. This audience should be large enough for you.”

Harry swallowed, “Um, Sn- Professor Snape put some confinement charm on me, so apparently I can’t go to classes until Monday. Could I get something to work on to make up for missing it?”

Professor Sprout looked at him kindly. “Of course you can, Mr Potter. I will send some work down later. This might give yourself and Professor Snape the opportunity to build on your relationship too.”

Harry very purposefully avoided looking at Snape, who he had felt tense up next to him. There was also the matter of the low growl that was emanating from the man’s throat. Harry wondered for a second if the Herbology teacher was annoying Snape on purpose, usually she was very no-nonsense, not the kind to talk about feelings…

The hall was a little busier now, with groups of student strolling in and sitting at their house tables. With a sudden lurch, Harry remembered why he had been out of bed last night in the first place. He watched the first large group of Slytherins – about twenty of them - walk in together to sit at one end of the long house table.

Snape was still talking to the other staff, “-you may send any work down with any Slytherin or any other student coming to Potions. Be sure to send enough to keep him busy, I am going to be stuck with Potter until Monday. There are only so many floors that need scrubbing, and Potter is hardly capable of preparing anything more complicated than mashed flobberworms.”

Snape whirled away, heading for the Slytherin table and Harry only just remembered that he had to keep up. The Professor picked a place on the bench next to his sixth year prefects, who didn’t seem particularly perturbed. They did give Harry a curious look, but were soon distracted by a conversation with Snape over a proposed change to their patrolling duties.

Harry collected himself a plate of toast, hoping that if he ate fast, Snape might decide they could leave early. He felt more anxious as the seats around him filled up. He did allow himself a small smirk when he saw Malfoy sneering at him from down the table.

Unfortunately, Snape had decided to eat slowly this morning, probably because he could tell that being at the Slytherin table was making Harry nervous. The man positively lingered over his last half cup of coffee.

Harry was just starting to think that the joke had been a failure when he saw it happen.

The Slytherin prefects’ hair turned bright green. Both of them changed in the same instant, his red hair and her blonde did a synchronised colour change. Neither of them noticed to begin with, because they were both so focussed on what Snape was saying to them. It was only when the Professor looked up at them and dropped his teaspoon and his jaw that they noticed anything was wrong.

From that moment on, almost every ten seconds, another Slytherin head would suddenly flash into the same shade of emerald. The students on the other tables were howling with laughter, as the wave of green passed down the table.

Harry was delighted to see a frantic Malfoy conjure a hand mirror just in time to see his own slicked back hair dye itself from the roots down.

Then he felt a hand on his arm.

“Did you not think last night,” Snape started in a dangerous tone, “That it would have been wise to inform me of this, before it got to this stage. I would imagine, even if you had not felt the slightest prick of conscience, you might have a glimmer of self-preservation. Have you forgotten you will be staying in the Dungeons for a few days?”

Harry swallowed, trying to look as innocent as possible.

“Sir, how could you think I could-“

“So if I were to examine your wand right now, I would not find any evidence of a time-delayed colour change charm? Or a banishing charm to dispose of a bottle of Madam Mim’s Witching, Wavy hair dye?”

“Uh…”

“That’s what I thought. Luckily for you, neither of those are permanent, especially when spread so thinly. Unluckily, everyone in my house knows you are responsible and they do not take well to being embarrassed in public.”

Harry had been starting to notice the stares from around the table. If the room hadn’t been full of quite so many witnesses, or perhaps if Snape wasn’t so close as to be practically a human shield, Harry would have been in big trouble.

Snape stood up, pulling Harry with him.

“Come along then, Potter. Since you are so eager to start, you can begin your first detention before classes start.”

Harry scrambled around so that he could put Snape in between himself and the table of angry students. He looked over the top of the Hufflepuff and Ravenclaw tables to see Ron who was giving him a not very discrete thumbs up. Harry knew Ron wouldn’t be quite so happy if it was him who had four days of Snape lined up.

Maybe it wouldn't be so bad. Four days was hardly anything, after all.