Mapping star systems, Enterprise encounters an odd cube. Apparently solid, it senses the ship, moves to block it - and when Enterprise attempts to disengage, pursues while emitting dangerous radiation. Kirk is forced to order its destruction. He probes farther, to find the builders of the device. And he does, when the Fesarius appears. The immense flag ship of the First Federation and her captain, the enigmatic Balok, regard the destruction of the cube as an aggressive act, and determine that the crew will be imprisoned and the ship destroyed.