Bumbling law office file clerk Mary Lou Packard stumbles across what she believes is a murder conspiracy when she learns that six out of eleven members of a tontine insurance policy recently have died of heart failure. At the office of Dr. Thomas Wilson, whose father was a member of the tontine, Mary Lou details her suspicions, noting that the tontine is a "suvivorship annuity plan," from which contributing members can collect after twenty years, is now worth $225,000 and is due to be paid in a few days. Thomas is skeptical of her reasoning and grows even more annoyed when she breaks into the law office where she works to show him the files and they are both arrested. Thomas' snobbish fiancée, Jessica Gerald, is angered by the publicity surrounding his escapade with Mary Lou, and wants him to end his association with her. Thomas becomes further enmeshed, however, when Mary Lou discovers that another of the survivors, Arthur Clay, is dead. Clay's landlady sees Thomas and Mary Lou after they examine the body, and she reveals their presence to the police, who soon suspect them of supplying Clay with the poisoned cigarette that caused his demise. With the police chasing them, Thomas and Mary Lou track down the other survivors, including Dr. Fred Turk, a dentist; Carter Stevenson; condemned prisoner Joe Ellis; and Ed Douglass, who is impersonating his dead brother Will, the real member of the tontine. The case grows more mysterious as Joe succumbs to a poisoned cigarette and Douglass is stabbed to death. Dr. Turk's wife Claire anxiously tells Thomas and Mary Lou that her husband has disappeared and implicates Carter in the killings. Carter is ruled out as a suspect, however, when he, too, dies after smoking a poisoned cigarette. Later, Thomas, who has been jilted by Jessica for breaking too many of their dates, goes with Mary Lou to the hospital where Dr. Turk is hiding. There, they learn that Claire murdered the tontine members so that her husband would receive all of the money, and then was planning to kill him in order to inherit as his widow. After Claire is arrested, Mary Lou's boss, Mr. Wheeler, offers Mary Lou and Thomas a reward, and Mary Lou faints when Thomas kisses her.