Oh dear. Harry, Hermione and Neville have been caught out of bed after hours. That costs Gryffindor 50 points each and earns them a detention. [=McGonagall=] wrongly believes that they fed Malfoy a fake story about a dragon to get him into trouble. For the first (and not the last) time, Harry finds himself unpopular among the students around him. He's left feeling so bad by it all that he even tries to resign from the Quidditch team. He overhears Quirrell in an abandoned classroom. He seems to be protesting against someone who is acting threatening towards him, presumably Snape.

Harry, Hermione, Neville and Malfoy go to their detention. They join Hagrid at 11 o'clock at night and have to venture into the Forbidden Forest. Hagrid says that a unicorn has been attacked and they need to find the creature, pointing to stains of silvery unicorn blood on the ground. The group splits as they enter, with Harry and Hermione following Hagrid one way and Malfoy and Neville taking his dog the other way. Hagrid stops them and tells them to cover after they hear something, which sounds like "a cloak trailing on the ground". They continue on. Then something else approaches, which turns out to be a centaur. They meet the centaurs Ronan and Bane, who are friendly but provide no real information, apart from noting that the stars are giving them bad omens.

Neville sets off an alarm. Hagrid rushes over and instead finds that he was prank-scared by Malfoy. Furious, he gets Harry to swap places with Neville. Harry, Malfoy and Fang continue on alone and eventually come to a clearing where the unicorn's body lies. And there, a cloaked figure leans down over the unicorn and drinks its blood. Malfoy and Fang flee in terror. The cloaked figure advances on Harry but a centaur races in and chases it away. The centaur, named Firenze, tells Harry the forest is not safe for him and gives him a ride him back to Hagrid. The other two centaurs are angry with Firenze that he is letting a human ride on his back and interfered with what the stars and planets forecast.

Firenze explains that unicorn blood can save anyone's life but at a terrible cost. Because it is "monstrous" to kill a unicorn, drinking its blood will give them "a cursed life", unless they could drink something else to give them a proper life. He reveals he knows that the Philosopher's Stone is hidden in the school and asks Harry who might be wanting to steal it. With a pang, Harry realises that Voldemort must be after the Stone to bring himself back to power. Harry and Hermione return to the dormitory to tell Ron what they find out. He also finds that his Invisibility Cloak has been returned.