After DI Dick Romney is killed under the wheels of a tube train DI Walter Gambon, a widower with a teenage daughter Lily and DS Anne Hopkins are charged by naïve young CS Addison to locate the undercover officer in Dick's large scale drug investigation. Fifty grand has also gone missing which Dick gave to his informant though Walter himself has debts of his own and when Dick's widow tells Walter her late husband was expecting a windfall from an American uncle Walter suspects corruption. Identifying Dick's killer from the tube station CCTV Walter follows him home, makes an arrest, uncovers corruption involving the recently deceased undercover cop and finds the missing money - which comes in very useful for paying off his debts, especially when Addison believes his story that it will probably never be discovered.