Unless one of the members of the Justice League can learn to miraculously produce food from nothing, the whole world will starve.
An emergency situation is brought to Aquaman's attention, in Atlantis. An alien vessel is stealing all the plankton from the world's seas. The commander of the alien vessel, Theo Zappa, triggers an underwater avalanche, to prevent Aquaman from disabling the ship. On the verge of unconsciousness, Aquaman activates his emergency distress signal, summoning the Justice League of America. Zappa, known as the Zapper, rose to fame, and fortune, by stealing an invention that first erases, then rewrites memories. Zappa had the device miniaturized, and weaponized.
One night, Pajna Darr, an alien from the planet Kalyarna, appeared before Zappa. Due to pollutants, the plankton population of Kalyarna's oceans were nearing depletion. The absence of such a vital link in the food chain had the civilization of Kalyarna teetering on the brink of ruin. Darr offered Zappa anything he desired in exchange for the Earth's plankton. Zappa used the memory gun to rewrite Darr's memories, garnering the alien's power wand. Zappa then embarked on a scheme to steal the Earth's plankton for himself, for use as a blackmail bargaining chip against both Earth and Kalyarna.
On the road "searching for America", the Green Arrow, and the Green Lantern, ignore Aquaman's distress signal. The rest of the Justice League of America, though, soon rallies to Aquaman's side. Aquaman explains the crisis, and the dire consequences facing the planet. Aquaman, and Superman, return to the sea, to search for the plankton stealing ship. The Atom, and the Batman, seek out Zappa. The Flash, and the Hawkman, return to the Justice League of America's satellite headquarters. Superman locates the receiving station for the plankton plundering vessels.
En route, Superman must pacify two sperm whales, agitated over the sudden loss of food. Spying Aquman's approach, Zappa releases the plankton back into the sea, driving the fish in the sea into a feeding frenzy, centered on Aquaman. Due to their great hunger, the fish refuse to obey Aquaman's telepathic commands to disperse. Superman saves Aquaman from being crushed to death by thousands of starving fish. Zappa uses the power wand to conjure a school of jellyfish. Their mystic nature proves overwhelming for Superman. Aquaman, however, is able to easily best the threat.
Aquaman, and Superman, are too late, however, to stop Zappa from teleporting the plankton off Earth. Using the satellite's scanners, the Flash locates Zappa's ship. The Hawkman, with the Flash, pursue Zappa, in the Hawkman's spacer. Zappa uses the power wand to neutralize the radiation shield surrounding the Hawkman's vessel. Undeterred, the Hawkman manages to dock with Zappa's ship. Zappa, however, teleports away, with the plankton. The Atom, and the Batman, discover Darr, in Zappa's laboratory. Zappa's memory gun only had a temporary effect on Darr's alien brain.
The Atom repairs Darr's equipment, then teleports, with Darr, and the Batman, to Kalyarna. In gratitiude for replenishing the plankton in Kalyarna's oceans, Zappa is now the ruler of the planet. The Atom, and the Batman, storm Zappa's palace, and subdue Zappa. The attack on their monarch spurs the populace to riot. The Justice League of America arrives. Superman addresses the Kalyarnans. Superman reveals Zappa's self-serving criminality, then assures the populace that the Justice League of America will restore their planet's ecology. It's up to the Kalyarnans, however, to enact measures to preserve their planets' ecosystem. An action the populace of the Earth would be wise to follow.