Nero Wolfe is stunned when Archie Goodwin reveals evidence of his impending engagement. However, Archie is no less shocked when accompanying his so-called fiancée to a Christmas party by the murder of their host.
Archie shocks Nero by informing him of his forthcoming engagement to beautiful Margot Dickey. The announcement will be made officially at a Christmas party hosted by avant-garde designer Kurt Bottweil. To Archie's surprise at the party, Margot now intends to marry Bottweil, not him, but before the evening is over, the designer lies dead, poisoned after drinking his favorite liquor. Although everyone at the party is a potential suspect, immediate suspicion falls on the unknown actor in the Santa Claus suit who mysteriously disappeared during the excitement. After Archie returns to the brownstone, Wolfe reveals to him that he was the enigmatic figure in the Santa outfit.
Nero Wolfe is shocked by Archie Goodwin's revelation of a marriage license, supposed proof of his forthcoming engagement to the beautiful Margot Dickey. In reality, Margot has asked Archie to provide a fake license to prod Margot's employer, avant-garde designer Kurt Bottweil, into finally marrying her. When Archie later accompanies Margot to Bottweil's annual Christmas party, all seems to go well... until the designer drops dead from a poisoned glass of Pernod, his "secret, public vice." Although all the party-goers are considered suspects, immediate suspicion falls on the unknown man in the Santa Claus suit who disappeared during the excitement. However, Archie soon learns that "Santa Claus" was none other than Wolfe himself! Now the two detectives must unveil the true culprit without revealing Wolfe's embarrassing subterfuge.
- Archie is dancing with Lily Rowan at the Flamingo when Margot Dickey cuts in. She asks if he's considered his proposal. The next day, Wolfe tries to arrange a trip to view some orchids, but Archie, resenting the intrusion on his time, shows Wolfe a marriage license for himself and Dickey. A tense lunch ensues as Wolfe tries to come to grips with the loss of his best agent.
That Friday, Wolfe leaves the house, ostensibly on his orchid trip and driven by someone else. Archie goes to a Christmas party where Dickey plans to announce their engagement. Various characters are introduced, and Dickey reveals that she and Archie had arranged to fake their engagement to force avant-garde designer Kurt Bottweil into a proposal. Bottweil, with a glass of Pernot, gets up to make a speech. He proposes a toast, but his drink is laced with cyanide and he dies. The bartender, a man in a Santa Claus costume, has disappeared, leaving his costume in the elevator. Archie chases after him, and also goes through the wastebasket looking for the marriage license. Neither he nor the police are successful at either task, and Purley Stebbins stays behind to interview the party guests. Many have motives - Leo Jerome is losing his inheritance to Bottweil and Cherry Quon wanted to marry him. Jerome's mother lies to the police, telling them she didn't know about the cyanide kept in the house for gilding purposes, and the situation degenerates into a fight.
The next day, Archie reveals to Wolfe that the engagement is off, which makes Archie, presumably, a jilted lover and a prime suspect. Wolfe asks Archie to get a book from his room. When he does so, he finds a pair of white gloves. He realizes that they were Santa's gloves from the night before, and therefore Wolfe must have been Santa.
Wolfe explains via questioning that he had had lunch with Bottweil after picking up his suit and gloves, and that Bottweil had indulged in a quick drink from the bottle of Pernot afterwards. The gloves were not part of the costume and thus could be traced back to Wolfe. They discuss options and Wolfe announces that the only acceptable alternative is to find the murderer and present him to Mr. Cramer. Archie explains his deception with Dickey.
Cherry Quon visits the office. She knows Wolfe was Santa, and claims that Bottweil was going to marry her rather than Dickey, and that Dickey therefore must be the murderer. She wants Wolfe to provide evidence in exchange for not telling the police. Wolfe sends Saul Panzer off on a secret errand, and arranges a meeting with all the people from the party, and Cramer.
At the meeting, Archie again explains the deception and that the license was never found. Wolfe tells the group that Bottweil had found a vagabond to play Santa and asked Wolfe to buy white gloves. Wolfe also sent each guest a note accusing them and asking them to meet the writer. Many took the note to the police, but Quon ignores it entirely. Margot Dickey was the only one to respond without going to the police first, and she is arrested after appealing to Archie for help and desperately accusing Quon.
Archie takes flowers to make up with Lily Rowan.