It is mid-2268, shortly after the events of the Original Series episode "Return to Tomorrow." The Enterprise, under the command of Lieutenant Hikaru Sulu, is returning from a mission to deliver medical supplies to Deep Space Station C-15. All is routine until the Enterprise comes within about a light-year of the planet Mu Arigulon; the ship runs aground on a spatial distortion not revealed in previous scans of the system and suffers a momentary power cut.

The ship continues on to Mu Arigulon: at warp 4, the Enterprise should have been able to detect any distortions before hitting them, but they run aground on another anomaly. There are a few more physical injuries, but one officer falls into a coma... for no apparent reason. His brain functions are failing, and McCoy manages to stabilize him, but he's unable to determine a physical cause for the problem. When Chapel pulls his medical records, she discovers that he's an esper: one of those rare humans with an abnormal level of telepathic/psychic ability.

Then the security officers bring in four more comatose officers. Once again, McCoy cannot isolate a cause for their difficulties-and once again, all four of them are espers. Their brain signs are slowly fading, and if they continue to degrade at their current rate, in only days there will be nothing left of them....