Maggie joins C.A.T.S. (Covert Activities – Thames section) as a replacement for an operative killed when investigating a Russian trawler anchored in the Thames Estuary. Maggie, under cover as a chambermaid, makes a mess of her first assignment investigating an apparently run-of-the-mill case of adultery. Fred, under cover as a Wren, sets out to avenge Jenny's death in a cat-and-mouse game with the Russians, while Pru heads off interference from covert Department 6 by posing as a West German publisher.