Arsenic and Old Lace is a dark comedy about Mortimer Brewster, his fiance or wife, Elaine Harper (depending on whether you're referring to the play or the movie adaptation of the play) and his two spinster aunts, who have taken it upon themselves to murder old men as a way of sparing them from loneliness. The two unstable aunts serve the elderly bachelors with elderberry wine laced with arsenic, strychnine and just a pinch of cyanide, and their brother, Theodore (who believes he is Theodore Roosevelt) buries the dead bodies in the cellar.
There are a few differences between the movie and play, among which are:
1) The play is set entirely in the Brewster living room, unlike the movie.
2) The play is set in September while the movie is set on Halloween.
3) In the play, Mortimer Brewster was a drama critic, while the movie version had him as a writer of anti-marriage books.
4) Different actors played the part of Mortimer Brewster. In the play, the part was played by Boris Karloff, but Raymond Massey did the honors for the movie.
5) The ending was also different; in the play, Mr. Witherspoon takes the aunts to the sanitarium, but ends up being the aunts' final victim. In the movie, final scenes have the two aunts telling Mortimer that he's not actually related to the Brewsters after all. Mortimer is so happy that he kisses his bride before they go on their honeymoon. The final line, "I'm not a Brewster, I'm the son of a sea cook," was changed from "I'm not a Brewster, I'm a b*s***d."