Shadow scientist Werag creates the Sun Smasher, which can cause a sun to go supernova and destroy an entire solar system in the process. Meanwhile, it's the Orbots' birthday, but they wonder why Rob has been squirreled away for the past three days. They figure it must be for a present for them, and curiosity gets the best of them and Ohno hacks into the computer to find out what he's doing. To their horror, they see that he's working on a new team of Orbots.
Umbra sends Earth an ultimatum: The Galactic Patrol must unconditionally surrender, or the Sun Smasher will be used to destroy everything. Rob wants to immediately head in with Mighty Orbots, but Rondu questions whether even he is powerful enough to handle this. Instead, Rob runs analyses on the sun which the Sun Smasher has already caused to go nova. In an effort to prove their worth to Rob, the Orbots decide to go to the Shadow Star themselves. Once there, they are quickly attacked by an army of Shadowbots. After Rob learns what the Orbots have done, he sets out after them and is immediately taken captive by Umbra. Mighty Orbots is overwhelmed by the Shadowbots, at least partially because Ohno can't work the controls quickly enough without Rob. But when Mighty Orbots learns that Rob is captive, he gets the strength to defeat the Shadowbots. Unfortunately, he is quickly attacked by another robot, the Destructitron. The Orbots are able to defeat him as well, and they head into Umbra itself to find out how to stop the Sun Smasher. Umbra plays with their minds and uses their fears against them, first making them believe they're in a junkyard, then hanging over a vat of lava. Rob, seeing the plight the Orbots are in, manages to escape but is hunted down by a giant rat. He is cornered by short circuits a control panel, which not only chases away the rat but also frees the Orbots. Rob and the Orbots meet up and Umbra, furious and panicking, orders Werag to activate the Sun Smasher, even though it isn't fully charged. The Orbots combine into Mighy Orbots, catch up with the energy beam, and force it back into the Shadow Star, where it destroys everything, including Umbra.
Later, with Umbra gone, the Orbots find out that the Orbot schematics they saw were of an earlier version of Orbots that Rob never built, and that he never had any intention of replacing them. Instead, he shows them an electronic birthday card, and gives each of them a birthday cake.