The Big Guy is decommissioned in favor of Quark Industries' newest creation, Rusty the Boy Robot. But when an alien Annihilator threatens to destroy New Tronic City, it's clear that Rusty is not quite ready for prime time. The Big Guy is reassembled and joins the fight, with Rusty at his side. When the monster is defeated, General Thorton orders that the temporary alliance to become a permanent partnership.

The Big Guy and Rusty face off against the Legion Ex Machina's evil creation: A.R.G. -12, a.k.a. Argo. When a Nucleo-Protonic blast from Argo triggers a malfunction in Rusty's power core, the boy robot runs away, fearing he'll be disassembled by Donovan for being "out of whack." The Big Guy must bring Rusty back to Quark. Just as Big Guy catches up Rusty, Argo attacks again. The robotic team must work against the clock to defeat the bad guy before Rusty explodes.

A telepathy experiment goes awry and a Quark Scientist, Dr. Neugog, is transformed into a giant brain on spidery legs. Neugog uses his telepathic powers to drain the minds of the world's greatest scientists. Meanwhile, Dr. Slate accidentally uncovers the Big Guy's secret: There's a man inside the suit, namely Dwyane. She decides to keep the secret from Rusty, because the shock could crash his vital Human Emotion Grid. The Big Guy and Rusty are called into to stop Neugog, but Dwayne proves vulnerable to the monster's psychic attack and seeks a little hi-tech help from Slate.

Dwayne takes a furlough to attend his nephew, Jeffy's eighth birthday party. Rusty hears about it and decides he wants to see what a birthday party is like. Since you can't go to a party without a present, Rusty grabs a cool-looking gadget from one of the labs in Quark. Unfortunately, this particular gadget turns out to be a piece of stolen alien technology, known as a Dimetrium Agglomerator. When Emek and Naard, the aliens who own the Agglomerator, come to claim it, the party turns into a war zone. The Big Guy and Rusty must stop the aliens before they use the Agglomerator to destroy the Earth.

Dr. Gilder, the Legion Ex Machina's mole within Quark, has developed a personal grudge against Donovan. Gilder uses his robotic expertise to take control of a prototype and tries to assassinate Donovan. After Big Guy saves the day, Donovan is taken to a secret location (the S.S. Dark Horse) for his own safety. To locate Donovan, Gilder needs a new plan. He happens across Rusty and learns that the boy robot knows Donovan's whereabouts. He also learns that Rusty wants a new, Big-Guy-style weapon. Making nice, Gilder builds him one. But as soon as Rusty fires it, the gun jacks into Rusty's systems, giving Gilder total control. Gilder sends Rusty to destroy Donovan. In the final battle, Big Guy must stop a rampaging Rusty -- without destroying him.

Dr. Slate, Big Guy and Rusty uncover the Legion Ex Machina's latest fiendish plot: a mysterious "final assembly" taking place in a network of underground tunnels. When Big Guy and Rusty find the tunnels, they derail a robotic train. Inside the massive train's cargo hold is a gigantic hand, that boasts a built-in high-tech cannon. With their plan uncovered, the Legion Ex Machina members decide to activate their latest creation ahead of schedule and missing a hand. The gargantuan Argo 12000 marches Godzilla-style through downtown New Tronic, as Big Guy and Rusty struggle to stop him. Hoping to be a hero, Rusty revs up to super-speed and rams the robot, disappearing completely. While Slate and Dwaybe worry that Rusty is dead, Rusty comes to inside the giant robot. Meanwhile, rather than simply destroying the city, Argo 12000 has stolen the latest Quark patent, the ultra-efficient microfusion generator: a perfect power source for an army of Argo 12000s. Big Guy and Rusty must work together to detach the rest of ARGO 12000s forearm. When connected with the missing hand, it will form the only weapon powerful enough to destroy the giant robot.

When Quark's Dr. Kirby disappears from his experimental Quantum Machine, a hostile monster emerges goes on a rampage, apparently having destroyed the doctor. Back at Quark, Rusty discovers a precious "boy robot" prototype. Hoping for a new friend, Rusty reassembles the robot, who he names "Earl." Before Slate can warn him about Earl's aggressive program, Rusty is called to fight the rampaging monster. Facing the beast, Rusty intuits that the creature is really innocent. But after the damage this creature has done, no one believes him, not even Big Guy. Rusty defies Big Guy to protect his new friend. After Rusty's disobedience, the reactivated Earl is assigned to help the Big Guy and Rusty track the creature. Midway through the battle, Rusty is able to convince Big Guy that the creature doesn't mean any harm. But Earl's relentlessly aggressive programming turns him against the heroes when they try to protect the creature. In the final battle, Big Guy and Rusty face off against Earl, as Slate and the Pit Crew try to sent the creature back home.

On Quark Industries' recently christened space station, the computer, known as Edie, has gone mad. Holding the heavily armed station hostage, Edie demands delivery of the Ubik Chip. The chip would allow Edie to gain control of all computers on Earth. Edie's strict terms dictate "two humans, no robots," so Slate and Dwayne are sent to deliver the chip. What Edie doesn't know: The Big Guy suit has been disassembled and integrated into the delivery shuttle. Once docked with the station, Dwayne must reassemble the Big Guy while Slate makes the delivery. Upset that he can't be part of the mission, Rusty sneaks onboard the shuttle before lift-off. When Rusty's presence is detected, the mission goes awry. Edie threatens to detonate a bomb on the moon, destroying it. While Rusty struggles to deactivate Edie on-board the station, Big Guy battles Fabio, Edie's robot sentry, on the lunar surface. Our heroes must stop Edie and diffuse the bomb before it's too late.

Donovan unveils Quark's latest prototype: Nurds, tiny loquid metal robots that repair and even upgrade any technology they come in contact with. When Big Guy becomes infested by the Nurds, they repair Big Guy's original artificial intelligence circuitry, making Big Guy a fully functioning robot, and kicking Dwayne out of the pilot's seat. But shortly after the robot Big Guy successfully completes his first mission, things go awry. As Dr. Slater had suspected, the Nurds needed more testing. Big Guy malfunctions, going on a car-smashing rampage through downtown New Tronic City. When Rusty is unable to bring his hero down, Dwayne takes on a daring repair mission to stop Big Guy and save the citizens he's endangering.

Rusty's frustrated. He's been powered up for sic months and he hadn't grown an inch: he'll never "grow up" to be like Big Guy at this rate. Taking matters into his own hands, Rusty puts his robot head on the powerful robot body of Quark's T-7000 prototype. The result is 10-foot-tall "Russ," a sleeker and more coordinated version of Rusty. After several outstanding performances, General Thorton decides to split up the team: Russ is ready to work solo. But Russ learns there's more to crime-fighting than just being big when a gang of exosut-wearing chopshoppers (Gage, Big Rig, and Cargo) lure him into an ambush. The Chopshoppers meet their client, the Legion Ex Machine's Number Five. Just as they receive payment for the T-7000, Big Guy shows up to bust them. In the finale, Big Guy is locked in battle with Number Five, while Rusty (on a makeshift, Mack-made robot body) goes after the escaping Chopshoppers.

A small volcano erupts in New Tronic City. After Big Guy and Rusty contain the flow of lava, a more serious problem is discovered: an enormous pocket of magma had formed beneath the city. If it erupts, it will destroy New Tronic, and cover the Earth in liquid magma. The solution: Big Guy and Rusty must journey underground to the heart of the volcano, and deliver a highly unstable Cobalt Thorium-G Bomb. The chain reaction set off by the bomb will "put out the fire" before it can erupt. Delivering the nitro-glycerin-like bomb is made even bore difficult when they are attacked by magma creatures, who turn out to be the ones engineering the volcano, in an attempt to take over the surface world.