Looking at the title, those who have only seen the films will think there’s not much here that didn’t make it into the film. They would be wrong.

We start with the fact that it’s Neville in trouble for Gryffindor with Harry and Hermoine. Ron is back in the hospital wing.

Had Harry thought that things couldn’t have been worse? He was wrong. When Professor McGonagall appeared, she was leading Neville.
“Harry!” Neville burst out, the moment he saw the other two. “I was trying to find you to warn you, I heard Malfoy saying he was going to catch you, he said you had a drag—”
Harry shook his head violently to shut Neville up, but Professor McGonagall had seen. She looked more likely to breathe fire than Norbert as she towered over the three of them.

They get a good dressing-down from McGonagall. By the description, it sounds worse than when Ron and Harry steal the flying car in the next book. Harry feels guilty, too, that Neville was dragged into it.

The number of points they lose for Gryffindor puts the house in last place. Harry is treated like crap by his fellow Gryffindors after this.

It was a bit late to repair the damage, but Harry swore to himself not to meddle in things that weren’t his business from now on. He’d had it with sneaking around and spying. He felt so ashamed of himself that he went to Wood and offered to resign from the Quidditch team.

Wood assures him that Quidditch is the only way to win those points back, but the rest of the team refuses to even talk to Harry, only calling him “The Seeker” during practice rather than by name.

Then, about a week before the exams were due to start, Harry’s new resolution not to interfere in anything that didn’t concern him was put to an unexpected test. Walking back from the library on his own one afternoon, he heard somebody whimpering from a classroom up ahead. As he drew closer, he heard Quirrell’s voice.

Harry infers that Professor Snape has finally forced Professor Quirrell to help him steal the Sorcerer’s Stone. He goes back to Ron and Hermoine. The scene in the book is very similar to the scene in the film. Chastened by losing points from their last adventure, Hermoine wants to go to Dumbledore, but Harry points out they have no actual proof. They are reluctant to do anything, plus they are scheduled for detention that night.

The detention is served with Neville and Draco. The scene is much different than it is in the film. Without knowing the future, the changes that were made don’t seem so bad. However, down the line in the books the centaurs are way more involved in the story than they are in the films.

When Harry realizes they will also be with Hagrid, he gets a bit optimistic.

Harry’s heart rose; if they were going to be working with Hagrid it wouldn’t be so bad. His relief must have showed in his face, because Filch said, “I suppose you think you’ll be enjoying yourself with that oaf? Well, think again, boy — it’s into the forest you’re going and I’m much mistaken if you’ll all come out in one piece.”

Draco loses the bravado he normally demonstrates. He’s often thought of as a villain, but he’s as much a child who is a victim of the environment he was raised in, particularly by his father. This scene shows that, as do several others throughout both the books and films. Their job is to locate an injured unicorn in the forest. Hagrid assures them that as long as they’re with him or his dog, Fang, they will be fine. When Draco says he’ll go with Fang, Hagrid warns him that he’s a coward. So much for being safe. However, it’s Harry and Hermoine with Hagrid and Neville and Draco with Fang, rather than Harry and Draco together like it is in the film.

Hagrid seized Harry and Hermione and hoisted them off the path behind a towering oak. He pulled out an arrow and fitted it into his crossbow, raising it, ready to fire. The three of them listened. Something was slithering over dead leaves nearby: it sounded like a cloak trailing along the ground. Hagrid was squinting up the dark path, but after a few seconds, the sound faded
away.

They meet up with a centaur known as Ronan. The conversation they have with him, and later Bane, don’t give them much to go on. They actually have a lot to say, but it’s cryptic and foreshadowing and no one there realizes it at the time.

After Draco spooks Neville on purpose, Hagrid re-arranges the groups so he and Hermoine are with Neville and Harry and Draco are with Fang. This scene is the same as in the film, with Harry and Draco finding the deceased unicorn and seeing the being that is drinking its blood, then being saved by the centaur Firenze. Draco and Fang have run off, but Firenze wants to get Harry out of there quickly, so he has him climb up on his back. When the other centaurs see this, it horrifies them. The see carrying humans on their backs as being beneath them.

“What have you been telling him?” growled Bane. “Remember, Firenze, we are sworn not to set ourselves against the heavens. Have we not read what is to come in the movements of the planets?”

Firenze spirits Harry away. They have the conversation about what the unicorn blood does, but Firenze leads Harry to see that there’s a connection to Voldemort.

“Good luck, Harry Potter,” said Firenze. “The planets have been read wrongly before now, even by centaurs. I hope this is one of those times.”

Back in the Gryffindor common room, Harry tells Hermoine and Ron what happened.

“Snape wants the stone for Voldemort… and Voldemort’s waiting in the forest… and all this time we thought Snape just wanted to get rich…”

Ron freaks out that Harry uses Voldemort’s name so casually. Even Hermoine is uneasy, but reassures Harry that he’s got nothing to fear as long as Dumbledore is around.

When Harry pulled back his sheets, he found his invisibility cloak folded neatly underneath them. There was a note pinned to it:
Just in case.

Hmm… Even curiouser. My bet is that Dumbledore had the Invisibility Cloak.

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Next chapter (link): Harry Potter and the Sorcerer’s (Philosopher’s) Stone Chapter 16: Through the Trapdoor