He is immediately court-martialled, but he offers no explanations for his actions – neither to his defending counsel and friend, Commander Jeffrey Vallance (Frederick Jaeger), nor to his only son, Richard (Andrew Ashby).

Richard happens to know about the Spens Code and its creator: a former Naval Intelligence operative named Hugh Spencer (John Woodnutt), a friend of his father's, who lives in Braxtet.

In order to cut time, they sail through the Navy's Forbidden Zone in the Channel, where they encounter a boat with radio-controlled operations and a strange set of aerials mounted on top.

Bayard, a former Navy scientist, has perfected the system by basing it on a two-beam transmission, collected and amplified at a central control station; but he intends to sell it to a foreign power for his own profit.

In the Beacons, Wheeler stumbles upon Bayard's operation and briefly frees Spencer before they are both recaptured, and after a long chase through the area Richard, Lucy and Pete are also caught.

Also, the youths have left some clues about what they have learned about the Doombolt project, but even with this information the Navy is helpless: since the beacons are small, locating them in these extensive areas proves to be impossible, and Bayard has taken steps to ensure that the Fiddich Brae remains ignorant to the danger.

After a long, desperate run, she manages to contact Naval Headquarters and provide Hatfield with the necessary information to track down the Doombolt control centre: Cragfest Island, in the middle of the Channel.

The series ends with the beginning of a sailing tour on Commander Wheeler's yacht, where Pete's reservations about having a woman on board seem justified when Lucy inadvertently causes him to land in the bay.

[1] The series has become a nearly-forgotten feature in television history, although it was commented on in several recent reviews as being more enjoyable than other, supernaturally themed HTV productions, such as Children of the Stones and King of the Castle.