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'sdrug 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.