Wealthy James U. Sperling approaches Nero Wolfe to investigate Louis Rony, an admirer of Sperling's younger daughter Gwenn. Sperling wants Wolfe to find evidence that Rony is a member of the American Communist Party. Wolfe is reluctant since he believes Rony has connections to “Z”, a shadowy criminal mastermind who has crossed Wolfe's path before. Nevertheless, Archie Goodwin is dispatched undercover to Sperling's Westchester estate to see if he can discover any grounds to convince Gwenn to break off the relationship.