NOVA Laboratory robotics experts Newton Crosby and Ben Jabituya developed several prototype robots called S.A.I.N.T. (Strategic Artificially Intelligent Nuclear Transport) for the U.S. military. After a live demonstration, one of the units is struck by lightning arcing through the lab's power grid. This scrambles its programming and makes it sentient and it escapes the NOVA facility.
The robot finds itself in Astoria, Oregon, and is found by Stephanie Speck, an animal caregiver, who mistakes him for an alien. She takes the robot into her home, where she provides him with "input" in the form of visual and verbal stimuli and it eventually names itself "Number 5", being the fifth prototype produced. She eventually discovers that Number 5 was built by NOVA, and contacts them about the lost robot. While waiting for NOVA to arrive, Number 5 learns about death when he accidentally crushes a grasshopper and concludes that if NOVA disassembles him, he will die, and escapes in Stephanie's truck. However, NOVA uses a tracking device on Number 5 to corner him and deactivate the robot for return to the facility. During transport, Number 5 is able to reactivate himself and remove the tracking device, and flee back to Stephanie.
Because of these unusual actions, Crosby tries to convince NOVA that something has changed with Number 5's programming and that they should take care not to damage it in their recovery efforts so that he can examine them later. Ignoring Crosby's concerns Nova instead sends their security chief and three other S.A.I.N.T. prototypes to recapture Number 5 by force. Number 5 reprograms the 3 robots to act like The Three Stooges, and escapes. Stephanie is able to convince Crosby of Number 5's sentience but they find that NOVA has called in the United States Army to capture Number 5. To protect his friends, Number 5 leads the Army away and appears to be destroyed by a helicopter missile. Stephanie is devastated at the demise of Number 5 and Crosby resigns from NOVA.
As they drive away from the scene, Crosby and Stephanie are surprised to discover that Number 5 had hidden under the van, having assembled a decoy of himself from spare parts to confuse the warmongers. Crosby decides to take Number 5 to his father's secluded Montana ranch where there will be much "input" for the robot, and Stephanie agrees to come with them. As they drive off, Number 5 asserts that his name should now be "Johnny 5" based on the El DeBarge song "Who's Johnny" which had been playing on the van's radio.
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