The crew of the USS Thunder, NCC 70605-A, under the temporary command of Lieutanant Commander Alucard Vess continued their investigation of a mysterious alien station buried deep within one of Jupiter’s moons. The initial away team under Major Hannibal Parker was quickly followed by a second under Lieutenant Commander Nugra. Both teams encountered hostility when they arrived. Nugra’s team was beamed into a holding facility which they barely managed to escape, while at the same time Parker’s team was captured by the station’s three surviving denizens — aliens known as Thrallians who had been studying and experimenting upon humans for centuries in a bid to reverse the genetic mutations they had unwittingly caused themselves.
Meanwhile, on the Thunder, the rest of the crew were not having an easy time either. The Thrallians utilized their advanced technology to bombard the Thunder with a signal so strong it knocked out the Thunder’s computer. Commander Miles Unum formulated a plan to use the Thunder’s runabouts to push the Thunder away, allowing the Thunder to use it’s torpedoes to destroy the signal emitters. The away teams succeeded in escaping and returned to the Thunder just as the Thrallians converted their station into a battleship and began their attack. After a prolonged battle the alien vessel was disabled, but not before the Thrallians escaped in a small scout craft.