Chief Supt. Franks looks into a hijacking of a plane in Edinburgh where the pilot has vanished which could be linked to the disappearance of terrorist Gabrielle from Paris.

French intelligence send pictures that show hit man Constant Delangre acting as Gabrielle's bodyguard and she likely to be the mystery passenger on the plane that landed in Edinburgh.

Capt. Aubrey Percival from SIS takes over command of the case. Franks discovers Gabrielle was a Soviet agent but is disillusioned. A hit man is already on her tail in Scotland.

Chief Supt. Franks is under pressure from Special Branch while Captain Percival is playing politics. Franks finds warns Gabrielle that a hit man is on her tail.

Special Branch take over the protection and surround Gabrielle's home. Frank's men track the hit man down to a caravan but he murders two people and slips the net.

Assassin Constant Delangre is shot dead by a sniper before Special Branch arrive with the evidence that Gabrielle is housemate Agatha, is actually communist agent who will stop her leaving for Canada.