Nero Wolfe accepts an invitation to address Les Quinze Maîtres, an international group of master chefs, on the subject of American contributions to fine cuisine. The group is holding its quinquennial meeting at the Kanawha Spa resort (possibly based on the famous actual resort The Greenbrier ) in West Virginia, which forces Wolfe to suppress his loathingof travel and trains on the 14-hour train ride from New York City. As a courtesy to Wolfe, Archie has been invited to the gathering by Marko Vukcic, Wolfe's oldest friend and one of Les Quinze Maîtres, so that he can accompany Wolfe.
On the way, Vukcic visits Wolfe's Pullman compartment to introduce him to Jerome Berin, another member of Les Quinze Maîtres. Berin is the originator of saucisse minuit , a sausage whose closely guarded recipe Wolfe covets. Berin, however, is scornful of Wolfe’s request, and the resulting discussion leads him to an angry denouncement of Philip Laszio, another member of the group who serves an inferior substitute for saucisse minuit in his restaurant. Laszio also stole Vukcic’s ex-wife Dina from him and the position of Head Chef at New York’s Hotel Churchill from Leon Blanc, another of the master chefs. His passion inflamed, Berin threatens to kill Laszio.
The next night, at a welcoming dinner for Les Quinze Maîtres, Philip Laszio insults the host, Louis Servan, and his head chef when he criticises the cooking. Tensions are further increased when Blanc refuses to tolerate Laszio’s company and Vukcic begins to succumb to the charms of his ex-wife, who appears to be seducing him. After the dinner, a tasting test is held resulting from a challenge made to Laszio. The test proceeds thus: Laszio prepares nine dishes of Printemps seasoning, each missing a vital ingredient, and the nine master chefs present along with Wolfe are challenged to taste each dish to see if they can determine the missing ingredient in each.
Wolfe is the last contestant to taste the dishes, but halfway through he summons Archie into the private dining room where the tasting is taking place; Philip Laszio has been murdered, stabbed in the back and hidden behind a room divider. The authorities are called, led by Barry Tolman, a local prosecutor who also happened to be on the train with Wolfe and Goodwin. At Wolfe’s suggestion, Tolman compares the results of the taste testing on the theory that the murderer, either tense before committing murder or shaken afterwards, would be unable to accurately determine the dishes. Of the participants, Jerome Berin’s test is the lowest-scoring, and based on Wolfe’stheory he is subsequently charged with murder. This drives a wedge between Tolman and Constanza Berin, Jerome's daughter, who have been developing a romantic attachment with each other.
The next morning, Wolfe receives a visit from Raymond Liggett, Laszio’s employer at the Churchill, and Alberto Malfi, Laszio’s assistant, who want Wolfe’s help in securing a replacement for Laszio at the Churchill. Although Wolfe is scornful of Liggett’s request and refuses his employment, when Berin is arrested he is skeptical that Berin could be the murderer and sees an opportunity to get the master chef into his debt. Wolfe decides to investigate and exonerate Berin for Laszio’s murder. Wolfe learns from Lio Coyne, the wife of one of the guests, that she saw two men wearing the uniform of the serving staff in the dining room around the time of themurder, with one of them silencing another.
Consequently, Wolfe gathers together the African-American kitchen and serving staff and questions them. In contrast to the racist and abusive attitudes of the police and authorities, Wolfe is courteous, respectful and civil to the men, but they are nevertheless skeptical and uncooperative until he appeals to their sense of equity and justice, arguing that if they shield the murderer solely because of his skin colour then they are“rendering your race a serious disservice” and are “helping to perpetuate and aggravate the very exclusions which you justly resent”. Impressed by the speech, Paul Whipple—a waiter and college student—admits that he was one of the men in the dining room that night. The other man was notAfrican-American, however; he was wearing blackface. It is also revealed that Laszio himself had switched the serving dishes in a spiteful attempt to humiliate Berin upon his turn, explaining Berin’s low score on the test.
This information is sufficient to get Berin released from custody. Having accomplished his objective— to put Berin in his debt – Wolfe turns his attention to the speech he is to give. While rehearsing the speech in his room, however, Wolfe is wounded by a bullet, shot through an open window. Wolfe is only grazed by the bullet but is enraged, and returns his attention to Laszio's murder: clearly, the same person who killed Laszio tried to kill Wolfe, and Wolfe intends to deliver the murderer to Tolman. He initiates further inquiries, carried out mainly by Saul Panzer in New York, and later presides over a dinner for the remaining members of Les Quinze Maîtres composed exclusively of American cuisine. The Maîtres are overwhelmed by the quality of the dishes and Wolfe has the chefs responsible brought to the room to be applauded by the diners — all are black men.
After the meal and despite the handicap of the facial wound, Wolfe delivers his speech on American cuisine, and— to the surprise of the gathered masters — continues by delivering the evidence that will convict Laszio's murderer and Wolfe's assailant. He reveals that the murderer was Raymond Liggett, who secretly flew into West Virginia the night of the murder, disguised himself as one of the wait staff,and murdered Laszio. He attempted to hire Wolfe to cover his tracks and to subtly bribe Wolfe not to interfere; when Wolfe secured Berin’s release, he panicked and tried to shoot him. Liggett was aided by Dina Laszio, who he coveted; she betrays him and confesses her part in order to prevent arrest.
The next night, Wolfe and Archie depart for New York, once again on the same train as Berin, Constanza, and Tolman. While Archie helps Constanza and Tolman mend their fractured relationship, Wolfe reminds Berin that Berin is in his debt, demanding the recipe for saucisse minuit as payment. Berin is outraged, but is eventually shamed into providing the recipe.