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
The point of no return is when Claudius asks Ros and Guil to figure out what is wrong with Hamlet and they agree without hesitation.
Act II
There are many points when Ros and Guil are far from resolution, but perhaps the most striking is when the two are lying in the dark on the boat in Act Three, not even sure if they are still alive.
Act III
This takes us to the end of the play. Hamlet switches the letter so that it condemns Ros and Guil to death, and their fate becomes increasingly apparent.