A strange 'torpedo' is found on the Atlantic shore of a lonely island in the Outer Hebrides, and in the attempt to defuse it a surprising discovery is made.

The torpedo turned out to be a spy kit with an international flavour. A Finnish passport, British and Swedish money, U.S. services pattern revolvers and East German binoculars. Most mysterious of all was a fragment of sheet music which proved to be from a Hebridean lilt.

Nicolson visits the white shores and his discovery on a deserted beach leads him into a trap.

With Nicolson temporarily out of the fight Grant decides to upset the quiet comfort of the hotel.

Nicolson pumps Ian McLeod about one of the new arrivals at the hotel. He then takes a trip with Mary Somers while Grant meets a visitor.

What was Dr. Glenville doing in the islands? Who is the mysterious friend of Colonel Jamieson whom Grant recognised? Who killed Grant and why? Nicolson makes an expedition to the dark island while the trawlers wait, and there is an unexpected meeting in the black house.