Soon after Dorothy Wynant announces to her inventor father that she plans to marry, he goes on a mysterious business trip, promising to return in time for Dorothy's wedding. As the day approaches and Wynant fails to return, Dorothy worries, while her mother, Mimi, is frantic that her ex-husband is unavailable to give her and her new husband, Chris Jorgenson, more money. When Mimi goes to see Julia Wolf, Wynant's mistress, to ask for money, she finds her dead body clutching Wynant's watch chain. Meanwhile, sophisticated former detective Nick Charles and his wealthy wife Nora have come to New York for the Christmas holidays and become enmeshed in the case, despite Nick's protests that he is no longer a detective. Nora enthusiastically encourages Nick, and one evening he and Asta, their terrier, discover the skeletal remains of a body in Wynant's laboratory. The police suspect that Wynant has committed another murder, but Nick realizes that the body must be Wynant's because of a trace of shrapnel found in the leg. Nick and Nora give a dinner party, to which they invite all of the suspects as guests. There it is revealed that Mimi had been aiding MacCaulay, Wynant's lawyer, in exchange for cash. When Nick exposes Chris as a bigamist, thus making Mimi realize that she will now be free to inherit Wynant's money, she incriminates MacCaulay, who had been embezzling from Wynant with Julia's compliance. Finally, Nick and Nora and Dorothy and her new husband Tommy are on a train, happily bound for California.