The adventures of three agents from outer space who are sent to Earth to determine whether the planet, a potential threat to the universe, should be destroyed. The instrument of destruction is a device resembling a large black ball with two antennae that is variously called an anti-proton bomb, a solar bomb, and a neutron bomb. Although the three agents (Captain Bokko, Nokko, and Pukko) are originally humanoid in appearance, upon arrival on Earth they take on the appearances of a rabbit (Bokko), a horse (Nokko), and a duck (Pukko) that they had captured as examples of Earth life forms. While on Earth they travel in a tire-shaped vehicle capable of enormous speeds called the Big Wheel, which can travel on both land and water (and, with modifications, through the air). The series tackles a number of issues which were surprisingly progressive for a manga of that period; particularly ecological concerns and poverty.