The Doctor and Jo visit the Master in his island prison, where he is given every luxury except his freedom. But there is something going on out to sea. Three ships have gone down with all hands in the recent past, and the Doctor takes it upon himself to investigate.
What undersea menace is at work? And how is the Master involved?
The eponymous race of this story have the misfortune of appearing in a story with the Master, and are largely relegated to the role of being "the muscle". This is unfortunate, as it has set the tone for all subsequent appearances. (The Sea Devils' "cousins", the Silurians got to hold the spotlight in their first showing and have held it ever since!)
The novel itself has the distinct advantage over the video in that the original TV serial was overlong, and the novel avoids spending too much time on running around. However, the straightforward adaptation of this story compares unfavourably with Mr. Hulke's adaptation of 'The Silurians'. There are also illustrations of a fairly poor quality.