Professor Robert Crater and his wife Nancy are archeologists, investigating the ruins of a civilization on M-113. Enterprise visits for the annual physical examination required by regulations, but the professor seems oddly reluctant. He insists all he and his wife need is salt to cope with the heat. Then Crewman Green dies, his face marked by odd red rings. Crater says he ate the borgia plant which contains toxic chemicals. But its symptoms do not include the red mottling. Kirk and Spock are determined to discover what's going on, and doubly so when a crewman on the ship dies the same way as Green.