In due s'indaga meglio

Three of the stories in the original "Partners in Crime" mystery story anthology were not made into TV shows; they were "The Adventure of the Sinister Stranger", "Blindman's Buff" and "The Man Who was No. 16". These stories comprise an ongoing case that spans the anthology. The introductory story, "A Fairy in the Flat" not only has the Beresfords asked to take over The International Detective Agency, but reveals that the agency's former manager, Theodore Blunt, was a part of a spy ring, and the Beresfords are tasked with intercepting coded messages. The three unadapted stories find the Beresfords threatened by various spies and eventually, discovering the identity of agent No. 16. The book ends with them closing the detective agency and Tuppence announcing she is pregnant.

Followed by In due s'indaga meglio: Avversario segreto (1983)