Agatha Christie's Partners in Crime: N or M?: Part 1

SINOPSIS

The first episode of a second three-part adventure for crime-solving couple Tommy and Tuppence. Major Carter realises there could be a leak in the department and is not sure who to trust. He sends Tommy on a top secret mission, which he cannot even tell Tuppence about. Checking into a Cromer guesthouse, he must unmask a Russian spy and discover what has become of a scientist working on a nuclear bomb, who has disappeared from a nearby military base. Tuppence does not take kindly to being left out, and begins her own investigation.