The Arctic waters off northern Norway, an icy international border where the military forces of NATO and the Soviet Union play a cat-and-mouse game of surveillance and counter-surveillance. These same waters are prime fishing grounds and the destination for deep-sea trawlers of many nations. One such trawler is the Caistor.

The British trawler Caistor has disappeared in the Barents Sea. Martin Taylor, son of the Chief Engineer, waits for news in the ship's home port. After the official search is abandoned, a private expedition to Norway is being organised by Tom Silvers, brother of one of the crew, but both British and Soviet Intelligence are determined to suppress public anxiety. As the official inquiry is announced, a lifebuoy is found by a Norwegian fisherman...

The public inquiry into the disappearance of the trawler Caistor has been interrupted by widows of the crew blaming the Russians. Martin Taylor, more determined than ever to discover the truth, sets out to trace a letter sent home by the ship's radio operator—but Evans has got there first.

Martin now knows that the Caistor was equipped for spying. Hillmore, having uncovered Main's illicit operation, suspends him, unaware that Evans has gone to ground—with orders to kill Martin.

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In Arctic Norway, Martin has found an ally in local journalist Erik Starvik. Back home, Suzy is in hospital after being attacked by Evans—and the information he forced out of her has put him back on Martin's trail.