Star Trek - Doctor's Orders
In response to good-natured complaints about his command style, Captain Kirk leaves Doctor McCoy in command of the Enterprise. Kirk beams down to the planet 'Flyspeck' in order to facilitate its acceptance into the Federation. Kirk soon vanishes, leaving McCoy stuck with the ship against his will; regulations forbid him from passing on command to Commander Spock.

Kirk is nowhere to be found and to complicate matters, the Klingons show up, claiming to have a stake in 'Flyspeck' as well. It is later found that Kirk had been lost in the time stream, as one of Flyspeck's races do not fully live in linear time.
James T. Kirk (Fictitious character), American Science fiction, Leonard  McCoy (Fictitious character), Science fiction, Spock (Fictitious character)
Diane Duane
Diane Duane (born May 18, 1952) is an American science fiction and fantasy author, long based in Ireland. Her works include the Young Wizards young adult fantasy series and the Rihannsu Star Trek novels.