Dr. Abbott Reeves and his colleague, Dr.Takada, are euphoric that their experiment, a “Mandroid” pilot, has successfully returned from a time travel mission, bringing an authentic Roman shield with him. The Mandroid tells Dr. Takada that he remembers nothing about his voyage, removes his artificial arm, and asks Takada to repair its circuitry. When the Mandroid leaves for the maintenance station, Reeves tells Takada that it is time for the bionic man to be dismantled. Takada objects since the Mandroid is half human, but Reeves ignores his plea. Takada informs the Mandroid that Reeves had ordered his destruction, and they must escape. Guards stop them as they leave the compound. Reeves’s voice orders the Mandroid to kill Takada, but the robot overrides the command and kills the guards instead. In the fracas, Takada is mortally wounded. As he dies, he urges the Mandroid to stop Reeves. Reeves’s men try to capture the Mandroid, but he shoots them dead with laser guns embedded in his arms. He blasts a hole in the compound wall and escapes into a forest on a “mobile unit,” a rolling transportation vehicle. Elsewhere, Colonel Nora Hunter runs an experiment with a robot named “Spot,” watching it fly around the room. A security guard in her office building discovers that someone has broken into the building vents. As he searches for the intruder, the Mandroid ambushes him with gas, rendering him unconscious. The Mandroid finds Nora Hunter in her laboratory, and explains that, sometime ago, Reeves rescued him from a plane crash in the Mexican wilderness and grafted artificial droid limbs and armor onto his body. Nora believes that Reeves died five years earlier, but the Mandroid informs her that the scientist has prolonged his life with transplants. The Reeves Foundation funds Nora’s experiments, but Reeves has stolen her designs to use for his selfish purposes. The Mandroid promises to find Reeves and kill him, but the electrical circuitry in his cortex shorts out. As Nora tinkers inside his head, the Mandroid finally recalls his battle with Roman soldiers. Nora announces that she wants to travel with him to Mexico to learn what Reeves is doing with her designs. The Mandroid resists at first, but it becomes clear to him that his mechanical limbs are fragile, and Nora will keep him operational. At a Mexican bar, Nora requests the toughest guide to take her upriver, and Harry Fontana volunteers for the assignment. Harry is not happy when the Mandroid shows up for the ride but increases his fee and they are off. Another guide named Bayou Betty believes that Harry stole the fare from her, and launches another craft to chase his boat. As Bayou Betty shoots at them, Harry maneuvers his boat to dodge her bullets. Nora insists on piloting the boat, as Harry throws an oil-filled drum into the water. He shoots the drum, setting off a fiery explosion. Meanwhile, Harry’s engine seizes and the boat grinds to a halt. As Nora offers to work on the engine, Bayou Betty and her associate, Maurice, show up in a bigger craft and demand that Harry turn over his fares. The Mandroid sizes up the situation and shoots a torpedo that destroys Betty’s boat. Nora asks Harry for a liquor bottle, pours some alcohol into the engine, and drinks the rest. As the engine starts up, Harry wants to know the identity of his passengers. The Mandroid removes his poncho, revealing his body armor, and orders Harry to keep going. Reeves’s associate, Ray, floats past in another boat, discovers where Harry went, and leaves Betty and Maurice stranded in the water. In the morning, the Mandroid senses that they are getting close to Reeves’s compound and informs Harry that his services are no longer needed. The Mandroid and Nora plan to travel on foot through the jungle but the trail has been wiped out. Nora activates her robot, Spot, to find the best route. Ray catches up to Harry and demands Nora and the Mandroid’s whereabouts. However, Harry insists on money for the information, then sprays them with a fire extinguisher. Harry escapes, but the men return and lose control when they try to ram Harry’s boat. They end up in the water, and their motorboat explodes after it crashes into the rocks. As Nora and the Mandroid continue, they discover primitive tools belonging to indigenous natives. Spot reports they are only one kilometer from Reeves’s compound. Soon, they see the Mandroid’s wrecked plane and Nora insists on searching it for clues to his identity. However, the craft begins sinking and Nora is trapped. Harry shows up and agrees to save her for a third of their supposed treasure. As they travel up river on Harry’s boat, he claims to have known all along that they were treasure hunters, and he believes Spot is some kind of metal detector. The Mandroid is worried that Harry will figure out they are not looking for treasure, but Nora tells him they will deal with the problem later. The Mandroid accidentally falls overboard while looking at a family photograph Nora salvaged from his plane. Harry scours the river but the Mandroid has disappeared, and Nora makes the decision to continue without him. At Reeves’s laboratory, the scientist now possesses an artificial left arm and hand with weapon capabilities. When Ray reports that the Mandroid plans to return, Reeves orders Ray to stop him from reaching the compound. In the river, the Mandroid and Spot float to the surface. They journey on land, while Nora works on the stalled engine, and Harry tows his boat through the water with a rope. As Nora argues with Harry about his incompetence, he disappears below the surface. Before she can react, Mexican tribesmen capture her, and pull Harry from the water trapped inside a wooden cage. The Mandroid meets up with Takada’s son, Kuji, a ninja looking for his father. The Mandroid informs Kuji that Takada was his only friend, but Reeves killed him. The tribesmen, who resemble Neanderthals, take Nora and Harry to their village. The tribe leader prepares a ceremonial fire. When Harry insists on a last kiss from Nora, he slips some bullets into her hand and orders her to drop them in the fire, which creates an explosion. As they escape, Kuji appears and fights other tribesmen. The Mandroid arrives and scares away the rest of the warriors. The travelers steal a tribal raft, but the Mandroid cannot control the jet propulsion in his feet, and the raft crashes on a riverbank. On shore, Harry announces that he has had enough and leaves. Nora chases after him, and persuades him to continue by explaining the real reason for their journey. Soon, they discover the Mandroid’s mobile unit and know the compound is near. Nora sends Spot on a reconnaissance mission. Kuji talks about how his father and Reeves discovered time travel. Kuji’s revelation explains why the Mandroid has memories of ancient Rome, and why Neanderthals inhabit the Mexican jungle. As they start up the mobile unit, Ray and others attack the group but are soon overpowered. At night, the Mandroid asks Nora to dismantle him because he is a killing machine with no past and no future. He laments that killing Reeves will not make him whole again. She persuades him to reconsider. Spot returns, but shoots everything in sight. Kuji chops the malfunctioning android in half with his samurai sword. Reeves’s floating head appears as a projection in the sky, and warns the group to abandon their mission. Nora tinkers with Spot’s remains, and discovers the coordinates to Reeves’s compound. The Mandroid separates from the others. Nora tells Harry his job is over, but Harry agrees to see the mission through to its end. Kuji flies between the blades of a giant rotating fan and dismantles it, creating a way into the compound. As they search, the friends stumble on a room filled with Roman artifacts, and Latin documents. Nora concludes that Reeves plans to return to ancient Rome as ruler, and must be stopped. As the Mandroid demands the gates be opened, Reeves’s men fill the courtyard. Reeves’s voice orders them to hold their fire, and the Mandroid announces that his friends have raided the laboratory and destroyed Reeves’s work. Reeves’s men have captured the friends, but Nora insists that the Mandroid save himself. When Reeves demands he drop his weapons, the Mandroid removes his arm. While Ray prepares to annihilate the Mandroid with a laser, Kuji destroys the laser with a projectile. A gun battle ensues, and Reeves’s men flee. The Mandroid orders Reeves to surrender but he laughs, appearing as a younger man encased in body armor. Reeves fights the friends with the same weapons, but with twice the Mandroid’s strength. He pummels the Mandroid until he collapses, and traps the friends in an electrical field. The Mandroid awakens and neutralizes the field, but is killed from the exertion. The friends chase Reeves and find him boarding his time machine. As Reeves blasts off, Nora attempts to change his destination but fails. Harry swipes his hand across the instrumentation and changes the coordinates to prehistoric times. Abbott Reeves will now be the ruler of nothing, and the friends laugh.