Harry enters McGonagall's office as instructed. To his surprise, she doesn't know why he's here. He requests a private conversation and McGonagall casts a bunch of spells to ensure secrecy. After suppressing the urge to offer her a Comed-Tea, Harry tells her what he knows about the Sorting Hat's message to him. McGonagall is very surprised by this as she had expected him to try to blackmail her for information, and is also surprised that he's responsible enough to come to her with this information. They reflect briefly on the idea that a Gryffindor would not be so reasonable, while a Hufflepuff would. Harry also reveals that the Sorting Hat offered him Hufflepuff.
McGonagall reveals that the message was probably involved in the death of a student 50 years ago, and Harry suggests that they should figure out how to take the message off or quiet it. She is very grateful to Harry and offers to award him a ton of house points, but he declines because it wouldn't be fair and it would be better to keep the matter secret anyway. McGonagall remarks that Harry is worthy of Hufflepuff, which surprises him somewhat. She decides to give him a gift. She returns with a golden necklace and a pamphlet. She tells him that its function is secret and that he should pretend it is a "Spimster wicket" to treat "spontaneous duplication" as cover for its true purpose. Harry admires this idea as a competent lie. He asks what it really does, and she tells him it's a Time-Turner, and he can go back in time 1 hour per spin, so that he can go to sleep at the right time.
Harry flips out about this for many reasons, the first and silliest being that time-reversed matter looks like antimatter, so using it might nuke the country. He holds it away from him and screams about how dangerous it is, but McGonagall remarks that it can't be that dangerous if they give it to children at all. He isn't very reassured by this, given his low estimate of the rationality of wizards, and voices his fear of accidentally going back to the Middle Ages and preventing the Enlightenment. She explains that this can't happen because the time-turner can only be used for 6 hours per day. He then fears the possibility of breaking it and trapping the castle in a loop, but McGonagall reassures him that the effects of it breaking probably won't be that extreme. Harry suggests that they should give Time-Turners a protective shell, and McGonagall thinks this is a brilliant idea, that she somehow didn't think of before. Then Harry begins ranting about the philosophical implications of changing the past and altering everyone in the process, but she interrupts and clarifies that Time-Turners are unable to change the past. As a result, Harry downgrades his threat estimation from a time machine to a live nuclear warhead.
Harry now tries to figure out how the Time-Turner might work. He figures that reality happens to be self-consistent somehow even though it has time travel in it, so there's no need to change the past. He considers the scenario where his past and future self encounter each other, and guesses that he'll see the same things both times. McGonagall points him to the pamphlet and advises him to schedule in order to avoid seeing an alternate version of himself. Harry asks what happens if someone doesn't do that, and she vaguely responds that it can be disconcerting. He worries that it might destroy the universe somehow, she replies that she would've noticed if that happened, and then Harry screams about the Anthropic Principle. McGonagall laughs and compares it to that time he yelled about her turning into a cat. He replies that it's even worse than that because the Time-Turner doesn't just violate the laws of physics, it's also not Turing computable(it's kind of unclear how he figures this).
Harry then yells out another long-winded explanation as he realizes that the Comed-Tea must also work by sending information backwards in time, rather than causing funny events to happen, the funny event causes him to get an urge to drink Comed-Tea just before, therefore it can't be used for omnipotence. He then realizes that this is also how the whole prank that brought him here worked, and his future self must be the culprit. He keeps this one quiet, and McGonagall remarks that he should teach the Muggle theories at Hogwarts even though they're wrong, and after a few more pleasantries and promises Harry leaves her office. He reflects on the paradoxical probability that the prank, which depended on him having a Time-Turner, was the only reason he received one in the first place. He also considered that perhaps he would have gotten it anyway, but the loop would have been inconsistent for some reason if he had gotten it later.
Harry then launches into a long philosophical train of thought about how, unlike other scientific mysteries, this particular question of how the Time-Turner works might be unsolvable to him because his mind might not be able to simulate the action of a Time-Turner even in theory. He encourages himself by remembering that he did manage to solve the mystery of the Comed-Tea. Harry decides to go back in time 5 hours to start the prank and also fix his progressing sleep cycle. He still doesn't yet understand how he pulled some of it off, and is impressed that Time itself presented him with a prank that he might think of himself.
After going back in time, Harry sneaks into his dorm at 6PM and finds a gift to him. He opens the letter attached to it, which says that the gift contains the True Invisibility Cloak Deathly Hallow, passed down to him by his family. The author is anonymous but claims to have been friends with Harry's father, who lent it to him before he died. He puts the cloak on Past Harry and stays until everyone leaves so that he can keep Past Harry's quieting charm up and plant all the notes. He swears to his dorm mates in legalese that he has no intention of going through their things or pranking them. Penelope Clearwater, the prefect assigned to them, confirms that Harry's legalese is correct and coaches everyone that they will get lost and tells them to ask a painting when they do. Once everyone is gone, he puts the Invisibility Cloak in Past Harry's pouch and takes it out of his own.
He then sets up the rest of the prank by passing on the message he heard to the paintings, saying that it came from a gap upon a fiery abyss. He stole pies from the Great Hall and ran to Herbology. He tipped off Professor Sprout about the bullying and told her not to tell anyone about his tip. He then went there himself under the cloak to watch and throw pies, and reflected that he'd yanked Neville too hard and his behavior didn't really look right or normal when viewed from the outside.
