Nero Wolfe and Archie Goodwin arrive at a rustic trout fishing ranch in the Adirondacks because Wolfe has been invited to cook his famous trout dish for a visiting foreing ambassador, Kelefy. Along with Kelefy there is also his beautiful lascivious wife, his aide, two rival American oilmen vying for a contract with Kelefy's country, and the American ambassador Leeson with his wife.
Wolfe is irritated by the rustic backwoods ranch, so unlike his own brownstone house which he never leaves - usually. He complains he has lumbago and scurries off to his room when he can, leaving Archie to be sociable. This proves to be quite a task, as the guests are either antagonistic, bullying, sneaky or just lusting after each other.
The next day the men go fishing in their allocated and isolated parts of the river. Wolfe stays to cook the fish and argue with the catering staff. Archie goes fishing after the others return and finds the body of the American ambassador, Leeson.
Archi races back to tell Wolfe, but he just wants to go home and tells Archie to pretend he didn't see the corpse. Nothing doing, Archie breaks the news to the rest of the guests and the police are called in.
The police arrive and Wolfe is antagonised by what he sees as clumsy and intimidating police questoning. He storms off to his room with Archie. A procession of the guests come to visit to offer information, point the finger of suspcion, or to ask questions.
Ambassador Kelefy and his wife particularly want to know why Wolfe did not cook any of the fish Kelefy caught. Wolfe tells them it was just a whim, nothing more. To make sure this answer doen't change, Kelefy has his sexy wife give Wolfe an emerald ring as a 'gift'. This is a bad idea as it just wounds the pride of both Wolfe and Archie.
Wolfe waits until the District Attorney also arrives and informs him he will give out the name of the murderer if the DA will gather everyone in the living room for an audience. Wolfe refuses to give the information privately because his pride has been hurt and he wants public revenge.
With everyone around him, Wolfe tells how he was bribed with an emerald ring - but the poor quality of the bribe did not sit with his conceit and it riled him. He tells the reason he did not cook Ambassador Kelefy's fish was because they were not fresh - they had been caught the previous day and kept in the river. Then they were retrieved today to provide an alibi to give Kelefy time enough to kill Leeson, because Leeson had had an affair with Mrs. Kelefy.
As the police move to arrest Kelefy, Wolfe reminds them the man has diplomatic immunity. Kelefy thanks Wolfe for the dinner then he and his entourage leave the ranch and the country.
Wolfe and Archie return to their brownstone. As they sit in the kitchen with Wolfe watching Fritz prepare dinner, Archie reads the paper to them. The story he relates is the assassination of Ambassador Kelefy as he stepped off the plane in his home country, though it was unknown whether this was because he was a murderer or because he ruined the chance of an oil deal with America.