First published in 1973, these short stories are Blish's adaptations of the screenplays of various episodes from the original series. The episodes aren't sorted into books according to either chronological order or identity of screenwriter.
"Return to Tomorrow" (episode 51, season 2, screenplay Gene Roddenberry and John T. Dugan). Investigating a planet left lifeless by a long-ago disaster, ENTERPRISE's crew is telepathically contacted by a surviving intelligence: three of the planet's finest minds were stored in globes during a war that destroyed their species millenia ago, and they now exist as pure thought. Sargon, his wife Thalassa, and their former enemy Hanoch offer a deal: their knowledge in exchange for help in constructing android bodies.
"The Ultimate Computer" (episode 53, season 2, screenplay Dorothy C. Fontana from a story by Laurence N. Wolfe) is the AI M-5, which despite 4 previous failed prototypes is brought aboard ENTERPRISE and installed by its creator, Richard Daystrom, to be tested in a series of 2-on-1 wargames, with ENTERPRISE further handicapped by running with a skeleton crew. [Speaking as a programmer: gang, haven't y'all ever heard of running *small* tests before hooking the AI up to a starship with heavy weaponry?]
"That Which Survives" (episode 69, season 3, screenplay John Meredyth Lucas and Dorothy C. Fontana) In a change of pace, Spock and Scotty remain aboard and have their own problems to deal with: as the away team begins investigating an apparently empty planet showing signs of terraforming, ENTERPRISE triggers its defense system: the starship is thrown nearly a thousand light-years from the planet, while a mysterious woman appears periodically to kill with a touch, despite the fact that the crew hadn't made any aggressive moves, or even been issued a warning.
"Obsession" (episode 47, season 2, screenplay Art Wallace) Dovetails well with aspects of Kirk's character from "Conscience of the King" - someone who can be unscrupulous in what he sees as a good cause. In this case, when the away team encounters a deadly cloud-creature, Kirk identifies it as the thing that killed half the FARRAGUT's crew 11 years before. On the grounds that it is obviously intelligent and poses a threat to inhabited planets, ENTERPRISE gives chase - but how much of the captain's real reason is atonement for hesitating to fire on the creature as a youngster?
"The Return of the Archons" (episode 23, season 1, screenplay Boris Sobelman) ENTERPRISE, investigating the area of Beta III to try to resolve the mysterious disappearance of the ARCHON a century ago, learns that the culture of Beta III has some mysterious form of brainwashing that can even effect some members of away teams - and for some reason, their rigidly controlled society disintegrates into complete anarchy in periodic festivals. There's an underlying pattern, but what is it?
"The Immunity Syndrome" (episode 48, season 2, screenplay Robert Sabaroff). A giant space amoeba has destroyed the INTREPID, all hands lost - one of the Fleet's all-Vulcan crews. Spock, more than Kirk, deals with the threat in this one.