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
Thoreau sets up his cabin and fields next to Walden Pond.
Act II
Thoreau lives beside Walden Pond for two years.
Act III
Thoreau leaves Walden Pond and rejoins civilized society.