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
Man-ape, meet slab. Slab, meet man-ape. Evolve together.
Act II
Heywood Floyd, meet bigger slab. Bigger slab, meet Heywood Floyd.
Act III
David Bowman, meet even bigger, bigger slab. Bigger, bigger slab, meet David Bowman and turn him into a Star Baby.