For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Teddy in the cabin with his parents, his discussion with Booper, and his diary entries. We discover that something important will happen today, but we don't know what it is.
Act II
Teddy's discussion with Nicholson. In this segment, the largest thematic and philosophical elements of "Teddy" are explored. This is the meat of the work.
Act III
Nicholson is left alone; he then hurries down to the pool deck only to hear the sound of a girl screaming.