[These notes were made in 1984:]. Cashing in, no doubt, on the popularity of the Star Trek movies, Bantam have issued a major reprint of this, the earliest of the published original fiction. Blish is of course an established sf writer in his own right, and it shows in the quality of his plotting, just as his long association with the series and characters shows in his easy familiarity with the whole ST universe. A botched-up experiment on the transporter produces 2 Spocks - the original and a "reversed" clone who, luckily for our sanity proves also to be reversed morally and therefore can be destroyed with impunity. The Organians, who had clearly captured Blish's imagination, make their appearance: creatures of pure thought, they have been captured by a "thought-screen," in the midst of which Spock 1 and 2 have their culminating battle. Faithful, well-written, and very obviously the model for many of the efforts which followed it.