The devil-suited villain Mephisto sends his minions to drop a sleeping pill into Atlantis's oxygen-creation plant so that it will contaminate their air and put everyone to sleep, letting him conquer the city without resistance. Aqualad catches them in the act, and Aquaman arrives and helps corral the henchmen. Aqualad manages to crawl into the oxygen machine to get the sleeping pill and toss it away, but has trouble staying awake for the rest of the episode.
With his main plan foiled Mephisto and his followers attack Atlantis with their attack submarines, but after a battle between Aquaman and a brigade of sea life Mephisto's ships are destroyed. Aqualad finally falls all the way asleep.