there something compelling and strangely believable about this episode . from the very beginning and an atmosphere of tension be create by the knowledge that a certain planet be go to explode within a few hour . kirk and spock and mccoy have beam down to evacuate the inhabitant and all of whom seem to have leave already for part unknown and except for an elderly librarian . the librarian polite but cryptic advice about where all the citizen have go to be interrupt by a crisis in which all three enterprise crew member find themselves unexpectedly hurl into different era of the planet past . kirk find himself in a time period resemble 17th century england and while spock and mccoy be strand in a desolate and frozen waste . the intercutting between the two story and and the different hazardous situation the men find themselves in be superbly handle and with return to the present an unknown chance and while the minute be count down to the planet explosion . imaginative writing and fine act characterize this episode and with a touching performance by mariette hartley a a woman exile to the ice age and and ian wolfe a the urbane librarian . somewhat reminiscent of the classic episode city on the edge of forever and this time travel story be a rich and compelling finale to the series and which conclude one episode later . this have to be one of the best of the whole series and especially remarkable give the generally less quality of the third season overall .