The asteroids are glowing, like nothing the crew has ever seen before. Captain Kirk orders the navigator to set a course to the nearest one, but upon reaching it, the Enterprise becomes caught in a electronic field shielding the planet. Kirk leads a landing party to the planet to discover the source. On board the Enterprise, Spock and Scotty manage to free the ship from the force field, but find that they cannot beam up the landing party without bringing the convicts with them. Furthermore, Spock discovers that the asteroid will explode within 24 hours.