The librarian's polite but cryptic advice about where all the citizens have gone to is interrupted by a crisis in which all three Enterprise crew members find themselves unexpectedly hurled into different eras of the planet's past. Kirk finds himself in a time period resembling 17th Century England, while Spock and McCoy are stranded in a desolate, frozen waste.
The intercutting between the two stories, and the different hazardous situations the men find themselves in is superbly handled, with return to the present an unknown chance, while the minutes are counting down to the planet's explosion.
Imaginative writing and fine acting characterize this episode, with a touching performance by Mariette Hartley as a woman exiled to the Ice Age, and Ian Wolfe as the urbane Librarian. Somewhat reminiscent of the classic episode City On The Edge of Forever, this time travel story is a rich and compelling finale to the series, which concluded one episode later. This has to be one of the best of the whole series, especially remarkable given the generally lesser quality of the third season overall.