The story described in these three novels (Present Tense, Future Imperfect, and Past Prologue published together in this Science Fiction Book Club Edition) takes place immediately after the Enterprise's visit to Psi 2000 (The Naked Time). The Enterprise returns to Tlaoli - an apparently "safe" planet - where a survey team had been left, the crew hoping to put the re-gained three days to good use. Strangely enough, even though the planet was initially deemed peaceful, uninhabited, and safe, the ruins are several spaceships are found. The Enterprise also encounters unexplained power drains and gravitational anomalies. Exploration in the surface leads to the discovery of a cave system and an alien device who's function is initially unknown. Enterprise crew members and then unpredictably plucked from the planet and sent off to other times and places. The timeline is altered and the Enterprise crew must figure out how to use the alien device to return their missing crew members and restore the timeline to its original history before the 'bubble universe' collapses and the timeline remains changed forever...
This is one of those books that is completely contrived - the problem and its solution completely rely on the features of the alien device (which really does not follow a logical set of rules - the device behaves one way to create the problem and then in an entirely different way to solve it). Several features of the device also do not seem practical at all, and why it would be designed to work in such a way that causes the caves it is in to freeze and become bone-chilling cold is again just a contrivance to throw challenges in front of our characters. While some of the characters are done quite well (e.g. Sulu, Uhura) others do things they would never do (e.g. Spock, Scotty). Scotty is unnecessarily cruel to Kevin Riley and Spock is poorly portrayed throughout.
Anyway, enough time spent on this book and on this review! Sadly, I cannot recommend this one!
At 439 pages and with pleasingly numerous references to events in the original 72 episodes of the first TV series, this novel is for hard-core Trekkies only. At MINIMUM you should watch "The Naked Time" which ends literally as this book begins, and "Arena" which is essential to understanding a large segment of the plot of "Janus Gate". The story-line is complex (at least to me) but does drag at spots (cutting 1/4 of the text wouldn't hurt). That said, the main characters are portrayed accurately, we learn about events in Kirk's boyhood, and we get to meet young Ensign Chekov (who is so new to the Enterprise crew when the story takes place that he doesn't even know his way around the ship yet) as well as his and Sulu's "older selves"(!). All in all, worth your ST reading time.