Dr Smith is given a simple job to do by Don testing the ship’s electrical systems. “When you see the yellow light go on, hit the blue button. Then wait two seconds and hit the black button. Then wait three seconds and hit the green.” Naturally Dr Smith confuses the sequence with the result that the Jupiter 2 suffers an explosion.
The Robinson’s commence using their auxiliary power unit until the main electrical generator can be repaired. Meanwhile the robot starts feeling weary and run-down. Will pleads with his father to re-charge the robot immediately but is told it would put too great a strain on the auxiliary power unit. “It might even burn out the system and that’s a chance we can’t take.”
Will is heartbroken and that night at dinner – eaten by candlelight since they are on emergency power only – the whole family is down because the robot is ‘dying’. Penny plays a tape recording of the robot saying his last goodbyes before leaving to die alone.
Will forces Dr Smith to together go searching for the robot. When they find him he is lying on his back and as big as a house. Mustering barely enough energy to speak, the robot tells Smith and Will – “Because of atmospheric conditions I have experienced a molecular reaction. This has reversed my ionic flow causing my great increase in size.”
Will and Dr Smith enter the robot via a compartment door carrying Will’s toolbox. Their goal is to reverse the robot’s ionic flow by making their way to the heart section and effecting the necessary repairs to the robot’s internal diode timer. They encounter numerous obstacles along the way.
John and Don go looking for Will and Dr Smith and eventually enter the robot. Will succeeds in reversing the machine’s ionic processor and the robot begins to rapidly shrink back to size with all four still inside. The three men make it out first and use a tree branch to keep open the narrowing exit door for Will.
The robot returns to normal size. He is helped to his feet and is ready to function again.