The Earth is threatened by a gigantic energy cloud which calls itself Vejur. Kirk retakes command of the Enterprise to investigate and attempt to stop the Intruder from destroying the Earth.

A novelization, but a good one. It's on par with the better Foster novelizations, at least--worth reading for its own sake. The story's not bad, though not quite as good without the mystery. We get a better look at the motivations of the characters--Kirk especially--compared to the film, which is nice.

Notes

Roddenberry in the 23rd century

Roddenberry positions himself, in the preface, as a figure in the 23rd century who has produced (and is producing) a dramatized account of real events.

love instructor

Opinion is divided on whether love instructor is meant here literally, a person hired to teach about sex, or whether this is simply chauvanist language for boyfriend or lover. I agree with the latter interpretation--if, as some suggest, this was merely an expression of Roddenberry's idea of a libertine future in which one might hire an expert instructor in the art of sex, then that should have been a mundane enough relationship that it would not merit memorializing it by the naming of one's child.