Synopsis:

After finding the USS Exeter in orbit around Omega IV with all her crew dead Kirk, Spock and McCoy beam down to the surface.   There they find Capt. Ronald Tracey who explains that an unknown disease on the planet killed his crew but that the planet itself protects people from the disease's affect.   They are stuck forever on the planet which is currently in the middle of a war between the Yangs and the Coms.

Another "alternate earth" episode.   This one is pretty fun to watch although the end, which is supposed to be exciting and revealing, is really too unbelievable.   This episode definitely takes the parallel development to an illogical conclusion.