Act I of this farcical and macabre play takes place entirely in the Brewster house in Brooklyn where the two spinster aunts of Motimer Brewster reside. Mortimer, ironically, a dramatic critic, arrives with an announcement that he is going to wed Elaine, the minister's daughter. Of course, the aunts are delighted and ask Mortimer to bring her for dinner, but he declines, saying that he has to cover a play tonight entitled, Murder Will Out. "I'll bet I can write the review without even seeing it," he tells his aunts which turns out to be dramatic irony. Disparaging the plot of the play, Mortimer suggests how it will be played out, perhaps with a body in a window seat such as the one they have. When he opens the lid of the window seat, however, Johnathan discovers just that--a dead body. Horrified, he tells his aunts that there is a dead body in the window seat, but they calmly acknowledge, "Yes, dear, we know."
Of course, Mortimer is taken aback by this acknowledgement. So, Aunt Martha explains that they have gotten into the practice of helping the lonely men who come to their house to rent Mortimer's empty room find peace by having them drink elderberry wine laced with strychnine and "a pinch of cyanide." When Mortimer tells his aunts that they cannot give men poison elderberry wine, they say, "We don't stop you from things you like."
In the meantime, the black sheep of the family, Jonathan, whose face now resembles Boris Karloff, sneaks into the house with a Dr. Einstein, a fellow miscreant who has operated on Jonathan's face in order to disguise him since "things got too hot" in Chicago. They, also, have a dead body, a Mr. Spenalzo who recognized Jonathan when he and Einstein hitched a ride with him. When they discover the "Panama Canal" that uncle Teddy has dug in his belief that he is President Roosevelt, they decide that they will later put Spenalzo in this canal. But, for now, they place him in the window seat just recently vacated by Mr. Hoskins whom the aunts quickly remove while Jonathan and the doctor are at their car. They have told Teddy that the man has died of yellow fever and must be buried. After Jonathan places Mr. Spenalzo in the window seat, Mortimer surreptitiously returns to try to hide Mr. Hoskins only to discover that there is another man in the window seat. When he asks Aunt Abby about him, she says that he is a total stranger, and, to Mortimer's questioning, she indignantly replies, "Darling, you don't think I'd stoop to telling a fib?"
Mortimer groans as the act ends. So the major action is the discovery, hiding, and removal of bodies.