A Separate Peace begins with Gene Forrester, our narrator, visiting the prep school in New Hampshire he attended as a young man during World War II – the Devon school. Wandering through the campus, Gene makes his way to a certain landmark which he cites as the reason for his return: a tall tree by the river. Cue the music, because we're heading into one long flashback.
We jump to Gene as a sixteen-year-old during Devon's Summer Session, in the months before his junior year, standing before the very same tree (good to know flashbacks always work the same way). Among others, he's there with his friend and roommate Phineas, a.k.a. "Finny," and a quiet boy named Elwin Lepellier who is unfortunately nicknamed "Leper." Finny decides they should jump out of the tree; Gene complies but no one else does. The two boys now share this bond. They also happen to be roommates.
As the Summer Session continues, we see that it's not all peachy keen between Gene and Phineas. Finny is an incredible athlete, unbelievably charismatic and just brimming with sixteen-year-old vitality. He can get away with anything, rules-wise, because the faculty members are so enticed by his charm. In short, Gene is jealous. Gene assumes Finny feels as jealous of his academic abilities as he feels about Finny's athleticism. When Gene finds out that this isn't true – that Finny is in fact pure of heart, without enmity, Gene feels even more jealous, more fearful.
Meanwhile, the boys have formed a club that involves jumping out of the aforementioned tree every night. On the day that Gene experiences his epiphany about Finny's good character, Finny proposes that they make a double jump, both boys leaping out of the tree and into the river together. Up in the tree, as Finny ventures out on the limb ahead of his roommate, Gene "jounce[s] the branch." Finny falls, rather than leaping forward, and thus lands on the hard ground instead of in the water. He "shatters" his leg, which means sports are finished for this grand athlete, forever.
When Gene goes to see Finny in the hospital, he's hesitant – but Finny suspects nothing. He thinks he simply fell from the tree. Gene himself doubts what actually happened – did he cause the accident or not? Finny then leaves school to recuperate from his injury, and shortly after the Summer Session ends.
When the boys all come back to Devon for the regular school year in September, Finny is still at home healing. Gene stops to visit him and confesses. Finny yells at him. Finny doesn't believe Gene and doesn't want to think about it. One of the senior boys, a typical leader-type named Brinker Hadley, accuses Gene, perhaps jokingly, of having pushed Finny out of the tree in order to have the double room all to himself. And the tension rises. Meanwhile the boys are all dealing with the fact that it's 1943 and they would rather enlist in the military than waste their years studying Virgil. Maybe. They're rather conflicted about the whole thing. Anyway, when Finny does come back to Devon, he forms a fast attachment to Gene (both boys having decided to forget Gene's earlier confession), whom Finny begins training facetiously for the 1944 Olympics. Since Finny can no longer be an athlete, Gene becomes an athlete on his behalf.
Things continue in this vein until their friend Leper, who we've seen is a peaceful naturalist, joins the military because he's enticed by the notion of ski troops. Months later, Gene receives a frantic telegram from Leper, who has "escaped" (gone AWOL) from the army. Visiting his friend, Gene discovers that Leper has gone mad. And also that Leper, too, believes Gene pushed Finny out of the tree. Ouch.
Back at Devon, things continue to heat up. Brinker again accuses Gene of having caused Finny's accident, but this time in the form of a "trial," along with many of the other boys. Leper is dragged in as a witness, but before any conclusion is drawn, Finny, still on crutches, leaves in a tear-filled rage. On the way out, he falls down Devon's famous (and very hard) marble stairs, breaking his leg for the second time. Gene is horribly guilt stricken and meets with Finny before his operation. The boys establish to a sort of peace between them. Then, during surgery, Finny dies.
The remaining chapter or two is devoted to the older Gene's musings on peace, war, and enemies. You should check them out, along with all those other chapters we summarized for you. See you there.