When a covert Elasian mining operation interferes with a Federation geological expedition, Captain Kirk and the crew of the Enterprise are dispatched to mediate between the two parties. The Elasian Dohlman claims that the planet and its dilithium deposits are hers, but the geologists warn that if Rakatan Mons-- the galaxy's largest known stratovolcano-- should erupt while the Elasians are mining its flanks, no amount of dilithium will save them. While Uhura and a landing party strive to determine if the Dohlman's claims are true, Kirk struggles to keep the two sides from each other's throats, but that delicate standoff is shattered by the arrival of an Elasian armada and the eruption of Rakatan, which could well destroy everyone on both sides.