Twenty years previous, the Enterprise, under Captain Robert April, had visited the planet Libros III and found that the simian-like native Usildar were effectively enslaved by the slug-like Jatohr who had arrived from another universe and were planning on taking over this one. Lieutenant Una (also known as Number One, and who later became First Officer) led the landing party to investigate what was happening, but the rest of her team end up being banished to the universe where the Jatohr came from. Una gets help from the Jatohr scientist, Eljor, who had devised the ‘Transfer Key’ that allowed movement between the universes (he was disgusted by the treatment of the Usildar). Eljor transports the other Jatohr back to their home universe, and before he dies of his injuries he places the key in the hands of April with instructions to keep it secret. April has it hidden in a secret compartment on the Enterprise. Forward two decades and Kirk is captain of the Enterprise: There is an upcoming peace conference planned between the Federation and the Klingons after the Organians had put an end to hostilities, and Una is now captain of the U.S.S. Yorktown but requests a ride on the Enterprise while her ship is in for a refit. However, her real reason for being on the Enterprise was to steal the Key and head for Libros III (now in territory disputed with the Klingons) and try to rescue the members of her first landing party from their exile. She does escape from the Enterprise and makes it to Libros III and nearly dies trying to get into the Jatohr citadel, but is saved by Kirk and Spock who had followed her there. Unfortunately, the Klingons notice the Enterprise approaching the planet and send their own forces, and Una is unable to locate her lost colleagues in the alternate universe from Libros III, so she talks Kirk into allowing her to transport herself to the alternate universe so she can find them. Kirk and Spock escape from Libros III, with the Key, as the Klingons take over the citadel, but Kirk has promised Una he will attempt to retrieve her in 60 days. Unfortunately, Kirk’s yeoman, Lisa Bates, was actually a Tal Shiar spy, Major Sadira, and she steals the Key and escapes aboard the Romulan bird-of-prey, Velibor. [Timeline: the events when April was captain took place in October 2249; the events when Kirk was captain took place in 2267, circa Stardate 3950.1, several months after the events in the ‘Journey to Babel’ TV episode]