A mysterious criminal known only as Dr. Vulcan has been stealing secrets and inventions from Science Associates to sell to foreign powers, and killing the inventors while making the deaths look like accidents. His latest victim, Professor Millard (James Craven) barely survives when a control panel explodes, showering him with electricity. With the help of fellow scientist Jeff King (Tristram Coffin), Millard sets up shop out of town in an abandoned cave. He lets the world believe he is dead so that he can finish his new atomic rocket suit as well as discover the identity of Dr. Vulcan.
Jeff meets with the other members of Science Associates; Bryant (I. Stanford Jolley), Chairman ( Douglas Evans), Conway (Ted Adams), and Von Strum (Stanley Price). They are all concerned about Millard's death and want Jeff to look into it. He agrees, making mention that Millard had been investigating the other deaths and was convinced that they were not accidents, but the work of Dr. Vulcan.
Rocket Man rescues Glenda from the crashing car, but Dirken gets away with the important negative. Later, at the conference, Jeff sets up a plan to trap Dirken while he is attempting to get a special developer for the enlargement of the negative. Dirken gets away with Vulcan's help, but Jeff, as Rocket Man, traces him to a hideout in the country. Here the negative is being develped which might reveal the identity of Rocket Man. Jeff, in an attempt to prevent this, is captured and held prisoner while the picture is being developed right under his nose.
Jeff forces a fight just before Rocket Man's identity can be disclosed through the print, which Jeff destroys as Dirken escapes. He remembers an ornate ring worn by an undisclosed confederate the night the photo developer was stolen. At a conference meeting next day, Jeff seeing the identical ring on Prof. Conway's finger, accuses him of complicity. Alarmed, Conway flees to phone Dr. Vulcan, whom be believes to be an innocent member of the project. Vulcan pretends to help and sends Conway to Dirken, who forces the professor to write a confession implicating himself as Dr. Vulcan. Jeff, as Rocket Man, flies to Dirken and Conway. Conway identifies his ring as a fraternal club ring worn by many Science Associates. Dirken kills him and escapes out the window, swinging across a suspended phone wire to an adjoining roof. As Jeff pursues, Dirken severs the wire, sending Jeff plunging to his death.
The severed cable swings Jeff back to a rooming house ledge. Returning to Science Associates, Jeff locates Vulcan's concelled microphone, through which he tricks Dirken into a secret rondevous. Burt and Glenda, misunderstanding Jeff's motives, intercede. Dirken is frced to take off with Glenda while Jeff subdues Burt. Vulcan's radio detects the direction finder Jeff planted in Dirken's plane and Dirken throws it overboard. Jeff gets rid of Burt and Pursues the plane in the rocket suit. Dirken parachutes to safety. Rocket Man gets into the diving plane and it crashes.
Rocket Man dives out of the plane with Glenda before it crashes. Dr. Vulcan now assumes that Jeff and the Rocket Man are one and the same and sets up a plan to prove it, stealing the newly devised Sonutron at the same time. At the conference, Jeff is put in charge of transporting the Sonutron and as he sets out on this mission he is captured by Dirken. Through a ruse, Jeff contacts Millard while enroute to Electronics Corporation. Dirken is about to consumate Vulcan's plan when Rocket Man suddenly appears. In the action that follows his surprissing arrival, Jeff makes an attempt to get away with the truck on which the Sonutron is loaded. A chance bullet knocks him cold and the truck crashes through the wall of the warehouse and off the pier into the water.
Jeff leaps to safety just before the truck crashes. He returns to the cave with the pseudo Rocket Man who turns out to be Millard. The professor demonstrates the powerful Decimator. Later, feeling his moves are being watched by Vulcan, Rocket Man sends Glenda and Burt to the cave with the Decimator and radios Millard to this effect. His message is intercepted by Vulcan who effects an attack on Jeff and sends his men to the cave to imprison Burt and Glenda and to fetch Millard and the Decimator. Jeff escapes and flies as Rocket Man to the cave. A fight starts with Dirken during which the Decimater is accidentally turned on, its force unleashing a lava flow through the tunnels. Rocket Man, Burt and Glenda soon find all escape cut off by the oncoming molten lava which overtakes them in one of the tunnel passages.
Rocket Man, Glenda and Burt escape through a fissure in the tunnel wall. Millard dies in stoping the Decimator. Jeff traps the hypnotized Von Strum stealing Millard's papers and delivers them for him. Dirken eludes capture. Vulcan send him to Glenda's apartment to silence Von Strum. Dirken and his henchmen hear the Rocket Man's approach and fire a volley of gunshots at him, as he soars up to the apartment window. He is apparently killed as the bullets shatter the window pane.
Rocket Man dives aside escaping the assassin's bullets, then drives them off. Inside Von Strum has been killed during the preceeding action. Jeff continues to work in secret on the Decimator but Vulcan has perfected a Thromium Detector by which Dirken locates the hideout and manages, through a ruse, to steal the Decimator. However, Jeff has concealed a camera which gives him a picture of the truck. The police spread a dragnet and locate the vehicle. Jeff as Rocket Man goes after it. In the meantime, Vulcan has arranged for a transfer of the Decimator to another vehicle and the seting up of an explosive charge in the truck. After setting up the detonator, Dirken is caught in the truck by Rocket Man. In the following struggle Dirken knocks out Rocket Man. leaving him unconscious in the truck as it is blown sky high.
Rocket Man hears the ticking of the device and leaps from the truck before it explodes. Back at the cave he removes his disquise, only to come face to face with Burt. Jeff is thus obligated to disclose the origin of the rocket suit, and how he came to use it. He reviews the incidents to date. Burt and Jeff now realize that Vulcan must either be Bryant or Gaffner, and plan to intensify their investigation. Vulcan, meanwhile, tells Dirken of his fear of Jeff and of his decision to draw Jeff into a trap. Through a ruse, Jeff is lured into a driverless taxicab on a false errand, and finds himself a prisoner in the air-tight passenger compartment. A poisonous vapor is relaesed into the cab through a floor vent and quickly overcome, Jeff slumps to the floor to meet his end.
Jeff is saved fromt he cab gas chamber by Burt and Glenda. He has gotten a clue that might lead to the identity of Vulcan. Vulcan is aware of the danger and sends Dirken to stop him. Jeff is about to find the incriminating paper when Dirken steps in and takes it away from him. A gun fight ensues during which Dirken escapes taking Burt along as prisoner. Vulcan decides to use Burt as bait to lure Jeff into a trap. The proposition is made and Jeff agrees on the rondevous set by Dr. Vulcan, who is planning an electrical trap to kill Jeff. Jeff, realizing the danger, arrives as Rocket Man ahead of the appointed time and is about to free Burt when Dr. Vulcan makes an appearance. Vulcan is now revealed as Prof. Bryant and he proceeds to back Rocket Man into the electrical trap, which erupts in a display of high voltage discharges.
Rocket Man is saved when Burt butts Gordon into the charged doorway. Bryant and Dirken, escaping with the Decimator, use it to start an earthquake and tidal wave off New York harbor when the mayor refuses to pay Dr. Vulcan's billion dollar ransom demand. Rocket Man flies to halt the Decimator's penetration of a volcanic fault in the ocean floor, unaware that a squadron of bombers is enroute to blast its island location off the map. Rocket Man destroys the Decimator, subdues Bryant and dives to safety just as the hut where they were fighting is blown to bits. Jeff persuades Burt and Glenda to allow the mayor to believe that he, not Rocket Man, saved the city from totla destruction and ended Dr. Vulcan's reign of terror.