To ferret out a possible mole in MI5, Det. Chief Supt. John Gaffney, Special Branch, undertakes a plodding, flat-footed (and authorially heavy-handed) investigation. The search begins with Gaffney baiting a trap--offering Army computer technician Armitage up for bribes--then sees it sprung when Russian agent Dickson, forewarned, scurries to safety--just as with two previous spy-drop stakeouts. With the assistance of Det. Chief Insp. Tipper, a homicide specialist, Gaffney interrogates the six-man MI5 team in charge of all three debacles; his inquiries lead to the suicide of section head Geoffrey Hodder and to the discovery of serious lapses in the MI5 vetting process, which missed several staff members' love affairs--including that of effete Peter Selby with Hodder's second wife Julia, a sex bomb with a sketchy background (vague ""charity work"" in Africa). Further plodding uncovers an alien spy ""safe house"" in Devon, nails a suspect at the Dover ferry, and more (supposedly) tough questioning reveals that the undercover agent was just about ready to pack it all in--for love. Surprisingly dull stuff, coming as it does from a retired Det. Chief Insp. of the Yard, who here shows no ""inner workings"" expertise or even dramatic flair.