This is book 16 of the "Nero Wolfe continuation" series.

This note contains spoilers for the book.

 

Horstmann is attacked by two people.

The title doesn't seem to be an actual reference to "trouble at the mill" (Monty Python).

These continuation books aren't being published in chronological order; this one is set around 1950, and the issue is smuggling in displaced persons (men to work, not women as sex slaves).

Horstmann has moved out of the brownstone, some months ago, and unexpectedly turns up, injured, on the doorstep. He's kept unconscious most of the book so Wolfe and Archie have to figure out what he'd gotten into by themselves.

They find he's been playing bridge with a group of other men, in a waterfront bar where they don't really fit in. And another of the group gets murdered shortly—the infallible clue in a Nero Wolfe story that they're on the track of something.