As a reward for helping irrigate their planet, an alien race bestows on Superman the honor of drinking the sacred scarlet nectar, which had not been bestowed for 25 years. Unfortunately, it had undergone a chemical change and become poison. Superman's invulnerability partially saved him, but instead caused a large weight increase.
Superman has an urgent deadline to return to his former weight, so he exercises hard, coming close by the deadline, but has to use a tool from another world to finish it.