Synopsis: In the 30th century, the Legion's powers are wildly fluctuating. Even Superboy is affected. The World-Wide Police suggests that, for the safety of the planet, the Legion be banished from Earth.
As their fate is being decided, the Legion is suddenly joined by a robot who claims he's behind the super-powered problems. Urthlo, as the robot is called, produces a device that he claims is behind the Legion's woes.
Thinking quickly, Superboy remembers Mon-El, whom he placed in the Phantom Zone some one thousand years before. Since he's been there the whole time, he would not be affected by Urthlo's device.
Superboy releases Mon-El, and Saturn Girl administers Serum XY-4. Mon-El is thus temporarily immune to his own chronic lead poisoning for long enough to defeat Urthlo.
In the end, the Legion discovers that Urthlo is merely an extension of Lex Luthor, and had followed Superboy to the Legion's time period.
Mon-El is restored to the Phantom Zone, but only after being voted into the Legion.