The Scarlet Beetle was an ordinary beetle mutated by the radiation of an atomic experiment. It gained conscious thought and the ability to command other insects. Receiving emergency impulses from his devices from his ants, Ant-Man decided to investigate what appeared to be problems in the insect world. There he found an irradiated beetle, able to communicate telepathically. The Scarlet Beetle planed to destroy the human race with the help of all the insects in his thrall (beetles, wasps, grasshoppers etc.)
Seeing Ant-Man as a potential obstacle in this plot, he ordered his insects to capture him, remove his helmet, and toss him in a pit that he could not climb out himself. Stealing Ant-Man's Pym Particles, the Scarlet Beetle exposed itself to Ant-Man's growth gas and grew to human size. He then ordered the insects on a campaign of terror, attacking humanity at all levels: knocking down telephone poles, stealing dynamite, and striking people ill with spider bites.
Meanwhile, although not on Ant-Man's head, his helmet sent out a signal that alerted his ants of his location, the ants arrived and helped Ant-Man out of the pit. Regaining his helmet, he and his army of ants faced off against the Scarlet Beetle's army of assorted insects, defeating them with honey, a water fountain, and DDT spray. Furious that his army was so easily defeated, the Scarlet Beetle was goaded into chasing Ant-Man into a nearby toy store. Inside Ant-Man managed to fight off the Scarlet Beetle long enough to secure a javelin from a toy knight and use it to pierce the shrinking gas canister that the Beetle wore around its body. Exposed to the gas once more, the Scarlet Beetle reduced back to his original size and was easily defeated by Ant-Man, who trapped the Beetle inside a deflated balloon. Having the Scarlet Beetle decontaminated of radiation, it resumed its normal intelligence and abilities, Hank Pym released it back into his back yard.