It is the first in the series to feature sailing (see also Falconer's Lure and especially Run Away Home) and the first to have a major male character.

Taking an early morning walk in a fierce storm, Peter Marlow is deliberately snubbed by one of his teachers from Dartmouth Naval College, Lewis Foley, who is spending the Easter holidays in the fishing village of St-Anne's-Byfleet.

Mariners turns out to be a deserted house complete with its own crow's nest and a sign to Foley's Folly Light.

Nicola discovers from one of her fishermen friends, Robert Anquetil, that Lewis had grown up in the village and that Mariners belongs to his family.

A man enters the cellars and the girls are afraid, but Peter recognises him as Lewis Foley and starts to tell him about the secrets he has found.

[1] Ironically, since the treachery in the English version implicitly involves England's Second World War enemies, it was published in German.

It was first published in German as Notsignal vom Leuchtturm in 1970 by Benziger, Zurich, and later as Die Marlows und der Verräter in 1974 by Ravensburger.

[2] In contrast to the original English version, the submarine is stranded in the fog and the fleeing crew is captured by the navy.