Devil in the Dark

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.

Synopsis adapted from TVmaze (CC BY-SA 4.0)