Arsenic and Old Lace is a comedy play about a drama critic, Mortimer Brewster, who discovers his sweet, elderly aunts, Abby and Martha Brewster, have been poisoning lonely old men with elderberry wine, believing it to be a mercy to "put them out of their misery". The chaos escalates when Mortimer's murderous and disfiguring brother, Jonathan, returns with his plastic surgeon accomplice, Dr. Einstein, and complicates Mortimer's efforts to protect his aunts and their deadly secret.

Key plot points
The Brewster family: The play centers on the Brewster home in Brooklyn, where aunts Abby and Martha live with their nephew, Mortimer, and their other nephew, Teddy, who thinks he is Theodore Roosevelt and spends his time digging the Panama Canal in the cellar. 
The "mercy" killings: Mortimer discovers that his beloved aunts are serial poisoners. They've been luring lonely men into their home and serving them wine laced with arsenic as a charitable act. 
The antagonist arrives: The central conflict arises when Jonathan, Mortimer's brother who has a long criminal past and a new, disfigured face from plastic surgery, returns to the house with his sidekick, Dr. Einstein. 
The struggle: Mortimer must now not only protect his aunts from the police but also deal with Jonathan, who poses a threat to everyone and represents a dark, criminal element to the family that the aunts' "mercy" killings have been trying to keep hidden.