While on a mission of mercy, the Enterprise comes across a shuttle craft stolen from Starbase 4. Its occupant is Lokai, a humanoid who is exactly half black and half white. Soon his pursuer, Commissioner Bele, arrives on board demanding that Lokai be turned over to him for transport to their home planet where Lokai has been convicted as a terrorist. Both men have extraordinary powers and it turns out that the pursuit has lasted 50,000 years. Their hatred of one another is racially based and, despite attempts by Kirk and others, they are not prepared to reconcile. The pursuit ends on their home planet where they learn the fate of their races.