David Mitchell takes the lead role of John Taylor, a man who enjoys spending leisurely days focusing on puzzles and word games. Keeping to himself in the home of his youth, John’s world is suddenly upended one evening with an unexpected phone call from Lucy.

Her husband James, John’s identical twin brother, has mysteriously vanished without a trace. A police detective, Lucy, suspects James’ disappearance relates to a case he had been investigating covertly. With no one else to turn to, Lucy recruits the socially withdrawn John for an improbable plan.

Posing as his missing sibling, John must infiltrate the police department in hopes of finding clues to solve the mystery of James’ whereabouts. What begins as a reluctant impersonation soon pulls John deeper into the complex casework as his logical problem-solving skills prove valuable to the investigation.

This six-part BBC series follows John’s foray into adapting to his twin’s life,...