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.