What is the major difference between the endings of Arsenic and Old Lace the play and Arsenic and Old Lace the movie?
In the movie, Jonathan is arrested and Einstein sneaks out of the house; and the papers are signed for the commitment of Uncle Teddy and the aunts to the asylum, Happy Dale. Elaine sneaks in and finds the 13 graves in the basement. As she "loses it," Mortimer picks her up and bodily carries her out of the house, kissing her to keep her quiet about what she has seen. In the movie they have already married so they leave ostensibly to go on their honeymoon, and the movie ends.
In the play, Elaine enters with Mr. Witherspoon. After the aunts tell Mortimer that he is not actually a Brewster (which alleviates his concern that insanity runs--"gallops"--in his family), delighted, Mortimer and Elaine leave to have breakfast.
In the play, both Jonathan and Einstein are arrested. However, when the police take the arrested criminals away and no one else is there, the aunts start to ask Mr. Witherspoon about his personal life. When they discover that he has no family, they go into action, inviting him to eat breakfast and have a glass of their elderberry wine. The audience assumes, as the curtain drops, that there will be another "yellow fever victim" in the basement very soon, and that--ironically--they will beat Jonathan's record in the number of murders he committed.