Maggie Forbes arrives to join the new unit on the day that team member Jenny Kenwright is killed while investigating a Russian trawler anchored in the Estuary. While Fred avenges Jenny's death in a cat-and-mouse game with the Russians, Maggie makes a mess of her first assignment – a run-of-the-mill adultery case.

When a cat burglar known as The Black Magic Man breaks into a Special Branch 'safe house' where two illegal immigrants are being held for questioning, Maggie finds herself on trial – but has to solve more than one mystery.