Even though Harry's worried about Voldemort, he continues studying for the end of year tests. He and the other first years endure their exams, which are both written and hands-on. Harry's scar is still bothering him, and he's having bad dreams again. Ron and Hermione aren't as worried.
They finish their last exam, in History of Magic, and the three friends go relax by the lake.
Harry tries to explain to the other two that he's worried and thinks trouble's coming They try to reassure him: Ron says no one can get past the monster dog. Hermione commiserates that she also feels like she can't remember something.
Harry's thinking and realizes that something important might have happened with Hagrid. He insists they all go see him immediately. He says it's weird and convenient that a stranger showed up with a dragon egg, which is something only Hagrid would really want.
At Hagrid's cottage, Hagrid welcomes them, and Harry asks about the stranger with the egg. Hagrid says he doesn't know what he looked like because he stayed in his cloak.
They ask Hagrid what he talked about with the stranger, and he reveals that he accidentally told him how to get past Fluffy – playing music will put him to sleep.
Freaked out, the three students return to the castle and start making a plan. Harry says they have to go tell Dumbledore right away. They're on the way when McGonagall stops them. She says Dumbledore's left for London after getting a message about important business.
Harry tries to get her to help by telling her that the Stone is in jeopardy. She's amazed they know about the Stone, but says it is safe and everything will be all right.
After McGonagall leaves, Harry says that they have to stop the plan that will be going down that night; he thinks Snape has masterminded the whole thing.
They run into Snape, who tells them to go outside, and warns them of possible expulsion.
Once Snape is gone, Harry makes a defensive plan. Hermione will lurk by the staff room and follow Snape if he goes anywhere suspicious, while he and Ron go wait by the room where Fluffy is.
But there they run into McGonagall. She says they have to get going, or she'll deduct major points. On their retreat, they run into Hermione, who was spotted by Snape and had to regroup.
Harry's had it; he says that night he'll try to get to the Stone too. He explains how dire the situation is and how they have to do something, or Voldemort will come back. All the other small problems will fade away if Voldemort makes a triumphant return.
Hermione and Ron say that they will help.
That night they hang out in the dorm until everyone else goes to sleep. Harry gets the invisibility cloak and the flute Hagrid gave him. They're getting ready to leave when Neville interrupts them. They don't want to tell him what they're doing, but he doesn't want to get them into trouble.
Hermione has no choice but to enchant Neville with a petrifying charm. They have to leave him, frozen, on the dorm floor and go on their invisible way to Fluffy's.
They pass Mrs. Norris and Peeves. Peeves can hear them, although he can't see, and threatens to call Filch.
Harry poses as the Bloody Baron and gets Peeves to back off.
At Fluffy's lair, the door's already open. Harry tries to tell the other two he can do it by himself, but they insist on going in as a team. When they go in, they see a harp and the alert dog.
Harry plays the flute, and the dog falls asleep. He keeps playing as they try to pass it.
They get to a trapdoor and can't see what's beneath. Harry jumps through and is followed by Ron. They land on some sort of plant.
Hermione follows and realizes that Ron and Harry are in danger – they've landed in a plant called Devil's Snare and are being pulled in by its tentacles.
Hermione escaped the plant and is panicking, trying to remember how to stop it. She remembers an Herbology lesson and realizes fire will stop it. Ron and Harry help her realize she can use magic fire.
The boys escape the plant and the three keep going into a passage. It reminds Harry of Gringotts.
They hear a noise and end up in a small room full of tiny flying things. There's a door on the other side but it's locked. Harry realizes the small things are keys.
They find broomsticks in the room and realize they have to fly to catch the right key, so they figure out which one is likely and begin searching.
Because Harry's a Seeker in Quidditch, he has an advantage. He spots the key and they attack it from all sides. The key struggles once they've caught it but Harry manages to insert it into the door and they get through as they key flies away again.
The next chamber is full of a life-size chess game. Ron realizes they have to play to cross. He says they should each take a position: he steps up as knight; Harry will be a bishop, and Hermione a castle.
Ron is in charge of the game because he's best at chess. He moves the pieces as the other two watch.
The three are freaked when they start losing pieces because it is so dangerous.
Ron realizes he has to sacrifice himself so the other two can cross. He says the other two have to keep going no matter what. He puts himself open to the white queen, who crushes him and makes him unconscious.
They win the game, and the other team leaves.
Hermione and Harry continue. They're trying to figure out what other spells to conquer. They pass a giant troll that has already been vanquished. The next room has a bunch of bottles – it's Snape's spell – in front of a row of fire.
Hermione finds a paper with rhyming clues on it next to the bottles. She says it's a logic problem, not a spell, and she can figure it out with the evidence it has.
She thinks, and says that the tiniest bottle will get someone through the fire, as Harry points out that there's only enough liquid for one. He tells her to go back, rescue Ron, and send for help, while he goes forward and tries to hold Snape off for a little while.
They embrace, and Hermione says to be careful.
Hermione drinks from a different bottle so she can retrace her steps. Harry drinks from the tiny bottle and goes forward. The drinks are like ice.
In the last room, Harry sees a person he wasn't expecting.