The story is set primarily in Dorset, England, from April to August 1984.

Lieutenant Stephen Lodge RN has been away from his home for more than ten months on HMS Alderney, a fishery protection vessel patrolling the North Atlantic. He is due to take extended leave when he returns but on their way back the crew are called upon to help the search for two missing divers in Scapa Flow. The diver's bodies are found and recovered, and an inquest is held, during which time Lodge and the Captain of the Alderney find sufficient evidence to suggest a third diver was responsible for his colleagues' deaths.

On his eventual return to the village of Ashfield, Stephen is confronted by nine-year-old Harri Jasper who says that a tramp seen around the village is none other than Nikolai Volkov, the KGB agent that kidnapped her in 1982 while seeking to avenge the death of a friend, Pyotr Petrovich. But no one believes her because Nikolai Volkov was sent to prison for the death of Mrs Hilda Smith. Before Stephen can investigate properly, an incident at the Libyan People's Bureau in London when WPC Yvonne Fletcher is killed recalls him to his specialist job for the Ministry of Defence and he is despatched to Tripoli to track down and eliminate Mustafa Ahmed, the person thought to be responsible. On return from the mission, he is surprised to find that Nikolai Volkov was in fact released in exchange for two British agents in August of the previous year.

When he finds out that one of Harri's schoolteachers has suddenly died, he investigates and is persuaded that her "tramp" could be responsible, but he is running out of Leave days and is posted as an instructor to HMS Vernon in Portsmouth. The tramp has now disappeared from Ashfield. While in Portsmouth and on a weekend visit from his girlfriend Jessica, an attempt at murder is made on their lives when an unrecognised Nikolai Volkov gives Jessica two barley sugar sweets laced with Ricin. She puts them both in her handbag and eats one on the train going home to London, where she becomes seriously ill from the effects of the poison but survives because she is able to contact Stephen's London boss Sir Geoffrey Cheeseman in time to get her into the University College Hospital. Johnny "Jack" Johnston, whom Volkov lodges with while in Portsmouth, is found dead at home a week or so later, another victim of Ricin poison.

While Jessica recovers and Stephen remains at her bedside in the hospital, another murder attempt is made, this time on Harri's father Roy when he is attacked by Volkov on the way home from the Greene Dragon pub in Ashfield. So begins a cat-and-mouse chase for Stephen Lodge and Nikolai Volkov through the countryside and villages of Dorset, England.