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
Buck is stolen from his home of comfort and forced into a life of hard labor as part of a sled team traveling in the frozen North.
Act II
Buck learns to adapt to his new environment, taking over as leader of the sled dog team, hardening his body, and getting in touch with his primitive roots.
Act III
Buck develops a closeness with Thornton until Thornton dies; then Buck joins the wild.