Harry and Hermione are in big, big trouble. Filch deposits them in McGonagall's office to wait for her, and Harry is super worried about how much trouble they'll be in.
Problems increase when McGonagall appears with Neville, who was also out of bed – looking for Harry to warn him about Malfoy.
McGonagall demands an explanation but Harry and Hermione are silent. She thinks they tried to trick Malfoy, and got Neville mixed up in it.
Neville has hurt feelings.
McGonagall is really mad. They all have detention and are fined fifty points each from their house.
That night, they're distraught about how mad their housemates will be – they've taken them from first to last place in one night, and it's almost the end of the year.
By the end of the day, everyone (except the Slytherins) is upset at Harry. Ron is the only guy who isn't mad.
Harry is desperate to atone and even tries to quit Quidditch, but Wood won't let him. The team needs him, but they're crabby too.
Harry resolves not to meddle or try to figure stuff out any more.
Hermione and Neville have it rough too. Harry, Ron, and Hermione try to buckle down, study, and forget about it.
Fast forward to one week before their big tests. Harry's leaving the library when he hears Quirrell whimpering and crying. He sees Quirrell come out of a classroom, touch his turban, and walk away, without seeing Harry.
Harry looks in the room after Quirrell leaves but doesn't see anything. He thinks, though, that Snape was probably in there tormenting him. He returns to the library and tells Ron and Hermione about it. They think it means Snape's getting closer to snatching the Stone.
Hermione says they should talk to Dumbledore about it, but Harry thinks they shouldn't get involved. They don't have any evidence, and they already have information they aren't supposed to have. He thinks they should stay out of it.
At the following breakfast, Harry, Hermione, and Neville, are notified that they will have detention that night. They meet Filch, who's already with Malfoy, downstairs at eleven PM and he takes them outside, lecturing them about how they're really getting off easily.
Filch takes them to Hagrid's – Harry's momentarily relieved until Filch says they'll be going into the forest. This scares even Malfoy.
Filch hands them over to Hagrid and says he'll pick them up at first light. Malfoy protests because he doesn't want to go in the forest or do menial labor, and Hagrid says if he wants to stay at Hogwarts he'll do as he's told.
Hagrid gives them instructions: this is a tricky situation. He's found unicorn blood, and he found another unicorn dead earlier. They have to stop whatever's hurting the unicorns. Hagrid says if they stay with him or Fang, they'll be safe. They'll split into two groups: Hagrid, Harry and Hermione will look in one direction, and Neville, Malfoy, and Fang will look another. They all have to be on guard.
Hagrid tells Harry that a werewolf isn't the one hurting the unicorns, and he doesn't know what else it could be. They spy more blood. Suddenly, Hagrid pushes the two of them behind a tree and arms himself. They hear a slithering noise, but don't see anything.
They move along carefully, and Hagrid calls out to something they see ahead to come out.
What appears is a centaur, Ronan, whom Hagrid knows. Hagrid tells Ronan what they're doing, and Ronan says some mystical things about danger and innocent victims.
Another centaur, Bane, comes in. He's not much help either, so the other three move on.
Harry is uneasy as they keep moving. Hermione spots a sign from the other group that means they're in danger. Hagrid runs after them while the other two stay. When he comes back, he explains that Malfoy played a joke on Neville, who panicked.
Hagrid is mad that they fooled around and changes the groups, swapping Harry for Neville.
Harry's group searches, following more and more tracks of blood, and finally they come across the dead unicorn.
They hear a creepy noise again and see a body with a hood slide across the ground to the unicorn and start drinking its blood.
Malfoy screams and he and Fang run for it. Harry's frozen and the thing comes toward him. He feels a searing terrible pain in his head and scar.
Something with hooves gallops up and runs at the scary hooded thing. The pain fades. Harry sees another centaur, who recognizes him, and tells him he must leave the forest. The centaur, Firenze, tells Harry to ride on his back. The other centaurs appear. They're mad at Firenze for letting a human ride him. But Firenze says that there are way more important things going on, and they just can't see it.
They ride off. Harry asks Firenze why they're so mad, and he explains that to take a unicorn's blood is one of the worst possible things. If you drink it your life will be cursed, and the only reason to do it is if you're on a quest for immortality.
Harry realizes that the person seeking the Stone must be Voldemort, and that must be what he saw in the clearing.
Firenze takes him back to Hermione and Hagrid, and the centaur says goodbye.
Back at the Gryffindor tower, Ron is sleeping and waiting for them. They wake him and tell the whole story. Harry explains that Snape must be working for Voldemort and the centaurs are worried about Voldemort's return, which they saw in the stars.
Hermione tries to reassure Harry, saying that Dumbledore will keep him safe.
When they go to bed, Harry finds his invisibility cloak hidden in his sheets.