Double, Double

Summary
From the back cover: On a routine exploratory mission, the Starship USS Hood picks up a distress signal from a research expedition thought lost long ago - the expedition of Dr. Roger Korby, one of the centuries' greatest scientific minds. Korby himself is dead, it seems, but his colleagues have made a most incredible discovery - a discovery they insist the Hoods captain see for himself. Reluctantly, the captain agrees to beam down...

Meanwhile, the crew of the USS Enterprise begins long overdue shore leave on Tranquility Seven. James T. Kirk is looking forward to a few days of rest and relaxation...until what seems like a bizarre case of mistaken identity plunges Kirk into a whirlpool of mayhem and murder.

And puts an inhuman stranger with his memories and abilities in command of the Enterprise.