The JSA has been beaten by the mysterious Fire People, so Dr. Grover Conrad and his assistant jack-hammer a message of warning on the side of the mountain for future generations. Ten days earlier, the arrival of yellow comets spur the JSA to investigate, and their encounter with a meteor sends them to Dr. Conrad, who explains one of the meteor's radiation printed a movie film from the long past revealing the arrival of the Fire People then, and their theft of radium, and the threat they pose for the future. The JSA decides to act and destroy the menace before it's too late.; Atom and Black Canary discover the Fire-People at a physics lab, stealing enough radium to power their get-away ship. After a short battle, the duo are chased off by the Fire-People's heat rays. Flash and Dr. Mid-Nite follow the footprints from a meteor to Five Lakes Hospital, and find the Fire-People in the surgery room. A battle ensues, resulting in their fiery foes using open ether cans to overcome them. Hawkman and Wonder Woman discover the Fire-People hauling away radium canisters from a mining company. They put up a great battle, but the duo is overcome by steam.; Green Lantern discovers where the Fire People are building their getaway rocket and notifies the Army that the enemy's radium gun is too much for him. The Army planes are also shot down by the radium gun and Dr. Conrad begins to chisel out his warning on the mountain, believing all is lost. As he finishes his message, the JSA shows up and tells him that the Flash has discovered the Comet People's weakness: HEAT! Then the JSA, armed w/carbon-arc lamps, attack the Fire People, and defeat them, who retreat to a hill and destroy themselves with the radium gun.; Hawkman illustrates 1) how the principle of flight works; 2) how to make a mailing-tube trombone, which explains the principle of sound; and 3) how a medicine dropper and balloon can be used to demonstrate how jet propulsion works.