When Tarka murdered his rival in love for Zira’s affections, he committed the first felony on Mars in 50 years. He was brought, along with Zira to face a court but before a sentence was pronounced he begs to make up for their deeds by fighting evil on Earth. The Martian government accepts the offer and branded his arm and sentenced him to exile. Together with his fellow criminal Zira, they were sent to Earth to rid it of crime. If they failed, then they would be destroyed–and so would Earth. Tarka took on the identity of Alan Wheeler, an FBI agent, with Zira as his secretary. They battled enemies such as the Black Hoods using futuristic weapons and their “disc-craft.” Sometimes they were called upon by Mars to battle interstellar criminals as well, such as the Uralian Pirates.

–Taken from pdsh.fandom.com