Miners on the mineral rich world of Janus II have begun to die horribly, burned to a crisp by the effects of a powerful corrosive. The Federation depends on these mines to supply minerals to many other worlds, so it sends Kirk and the Enterprise to investigate. But a starship, as one miner acerbically comments, cannot enter the mining tunnels. Kirk and Spock will have to use their wits to solve this problem. And they'll have to do it quickly, since the same agency has sabotaged the antique pergium reactor that maintains life support.