I distinctly remember being in my fifth grade science class and hearing a classmate talk about this movie. All I remember her talking about was the beginning, and what happened to Carrie in the shower, how she didn't know what was going on, and how the other girls made fun of her. That meant something to me at the time, I suppose, because I was in the middle of learning about the "joys" of womanhood myself. And I also knew what it was like to be made fun of because physical developments that were out of my control.
I can't imagine that there's anyone reading this who is not familiar with at least the basics of this story, but here's how it goes. Carrie is a misfit, in more ways than one. Her mother is a religious fanatic who believes that sex - even after marriage - is a sin, and that "women's troubles" are a judgement from God on a woman's sins. This has led to Carrie being a very shy and ostensibly weak person, as well as not being as "womanly" as the other girls her age.
One of the first scenes in the film is the famous shower scene, where seventeen-year-old Carrie gets her period for the first time. Because of her upbringing, she has never been told about what this is or that this is normal, so she thinks she is dying. She screams for help, but the other girls only mock her, throwing maxi pads and tampons at her and chanting "Plug it up! Plug it up!" The gym teacher intervenes and sends Carrie home, after explaining to her what happened. The girls who taunted her are given detention and threatened with refusal of prom tickets if they try to ditch. Naturally, this does very little to show the girls that what they did to Carrie was wrong, and instead makes them hate Carrie even more.
There is one girl (Sue) who actually has learned her lesson. Sue feels genuinely sorry for her part in the mockery and tries to make amends by persuading her boyfriend Tommy, the prettiest boy in school, to take Carrie to the prom. The other girls, however, are only concerned with how Carrie has ruined their lives. At the top of the totem pole of hatred is Chris, who has other plans for Carrie's prom night. Chris, her boyfriend Billy, and a few others rig the vote for prom queen and king so that Carrie and Tommy will win. Chris and Billy place a bucket of pig's blood over the stage, so that when Tommy and Carrie get up to be crowned, Chris can pull a rope and dump the blood all over Carrie.
Little do the girls know that Carrie is not as helpless as she seems. She turns out to have telekinetic powers. As she puts it, if she concentrates hard enough she can move things just with her mind.
The real meat of the story, though, is Carrie's relationship with her mother. Every one of the scenes between these two are bone-chillingly scary. After the mother gets a call from the school, telling her that Carrie has been sent home after the shower incident, she berates Carrie mercilessly. She starts reciting pseudo-religious propaganda ("the first sin was intercourse", "Eve was weak", etc.) and making Carrie recite it with her. She apparently believes that if Carrie had not done something to deserve it, she would never have been punished with "the curse of the blood." She then drags Carrie into the prayer closet to repent under the watchful eye of the most frightening crucifix you've EVER seen.
This is the best kind of horror movie - one that isn't just scare after scare but has a real story and believable, sympathetic characters that you care about. The big slow-mo shot where Carrie and Tommy walk up to the stage and are crowned king and queen of the prom is just heartbreaking. She's so happy, and you know what's about to happen to her. And when she snaps and starts killing everyone left and right, part of you is cheering her on. Then you really cheer her on when she finally takes on her psychotic mother.
Carrie is one of the great horror masterpieces. And the last 20-30 seconds of the film is one of the most skillful chair-jumper scares of all time.