Doctors Robert Crane, and Charles Grayson, are moments away from completing their greatest work, when the doorbell rings. Joan Carter has arrived, furious that Crane has forgotten their date. Carter storms off. Crane returns to his laboratory, when the doorbell rings again. Believing that Carter has returned, Crane is startled to see three armed men enter the laboratory. Word of Crane's invention has reached the underworld, and these hoods have come to steal it.
When Crane attempts to stop them, he is gunned down in cold blood. Grayson, however, is merely rendered unconscious. The three criminals discover a metal statue, that seems to hold no worth for them. Grayson, desperate to save Crane's life, transplants Crane's brain into the "statue", in reality an automaton. After completing the work, Grayson, exhausted, collapses. One of the crooks contacts the authorities, to blame Crane's murder on Grayson.
To protect the secret of what he has done, Grayson does not resist his arrest. Crane regains consciousness. At first, Crane is elated to be alive within the robotic shell, marveling at it's astonishing physical prowess. One glimpse of himself in the mirror, however, causes Crane's collapse, as the enormity of his new life, as a disembodied brain inside a metal shell, comes crashing down on him. The morning newspaper alerts Crane to Grayson's situation.
Crane attempts to hail a taxi, only to see the driver race away in fear. Crane learns that his robot body can actually run faster than the taxi. A crowd draws around Crane, heralding police intervention. A police officer fires on Crane, who realizes that his human brain is still somewhat vulnerable within it's metal shell. Crane flees, easily outpacing his pursuers. Crane returns to his laboratory. Using his scientific acumen, Crane creates a more human disguise for himself, that of "Paul Dennis".
As "Dennis", Crane visits Grayson in jail. "Dennis" poses as Grayson's attorney to gain access. Once alone with Grayson, "Dennis" reveals himself to be Crane. To clear Grayson's name, Crane decides to go after the real "murderer". Crane, as "Dennis", attends his own funeral. "Dennis" mets with Carter, intent on discussing Crane with her. When their taxi is sideswiped, "Dennis" subdues the aggressive driver. After parting with Carter, "Dennis" plays amateur detective, visiting various underworld hangouts, in search of Crane's "murderer".
A pair of lowlifes, promising "Dennis" information, lead "Dennis" into a dark alley, and attack him. "Dennis" easily overpowers the hoods, and gets the information from them. Removing his "human" disguise, Crane goes after his "murderer". Crane storms the hoods' apartment, and takes down the lot of them. Crane apprehends his "murderer", and forces him to confess to the Governor. Grayson is cleared, and released from prison. Crane decides to continue his crime-fighting career, as Robotman, intent on joining the Justice Society of America. Though Robotman never becomes a member of that team, he does become a charter member of the All-Star Squadron.