Amato and MechaBot are being chased by RocketFish, one of the most dangerous of the space prison captives. They get knocked down by his rocket-powered punch, so Amato has MechaBot mechanize with his circular multi hanger to form a machine gun, with ammo made from clips. The shots seems harmless, but they stick to the enemy and explode. Another rocket fist sends the two flying into a shallow river. Just as he's about to pummel them, he freezes at the sight of a larger robot on the bridge above them. MechaBot follows his gaze and is also fearful to see Sumorai. RocketFish begs Amato to capture him to avoid Sumorai's wrath. After Amato does so, Sumorai is gone.
MechaBot and Rubika provide Amato with more information on Sumorai, he's the most fearsome warrior in space prison, who was said to have defeated an entire army of robot soldiers and also destroyed their base prior to being imprisoned in the lowest level of the facility. They hear Mr. Gobi crying in his store, and also find Pian and Mara there comforting him. Mr. Gobi explains that a robot matching Sumorai's description made a giant hole in the back of the store wall. Amato and MechaBot track Sumorai, who throws a bin towards them before Amato can finish his sentence. He heads up and jumps across the rooftops, and the two pursue him to the town square.
Sumorai lands on a roundabout and slashes an incoming truck in half. Amato apprehensively approaches him to propose they take the fight elsewhere. Sumorai accepts and says they shall battle in the space prison arena before leaving. Inside the river's storm drain, Sumorai is distraught at having gotten himself tracked down and challenged into a fight. All he wanted was to buy a cat toy for Oyen-san, and even protected him from a moving truck. Sumorai doubts if he'll be brave enough for the fight, but Oyen-san wearing a box on his head gives him an idea.
That evening, MechaBot leads Amato, Pian and Mara to the space prison arena, and a masked Sumorai arrives. Mara sees through Sumorai's intimidation tactics, resulting in a one-upmanship duel between Mechamato and Sumorai. Oyen-san leaves Sumorai's pouch, Mechamato accidentally steps on his tail, and the cat scratches MechaBot's face before getting thrown off. Sumorai runs to attack him in retribution. At his cue, Pian and Mara throw him two flashlights, which he mechanizes with along with his shirt tied around his face to form the MechaNinja. With his rocket arms and stealth, he breaks Sumorai's mask as he launches him across the arena.
Pian and Mara find the "Oyen-san" that Sumorai keeps referring to, and show Amato his true nature by releasing the cat. Oyen-san runs towards Sumorai, but debris from above starts to fall. Mechamato jumps in the way, and Sumorai scatters the fallen debris, glad to find Oyen-san safe in the hero's arms. Sumorai explains that he used to be a soldier before he was modified for domestic use. The base he destroyed was due to a stove fire, and he asks to be put away. However, Mara reassures that it was an accident and he wouldn't be imprisoned unfairly. Sumorai is happy to be a helper at Amato's house instead, promising that he'll be more careful now that he has Oyen-san.
This episode provides examples of:
- Bare-Handed Blade Block: A variant. Mr. Gobi describes Sumorai, the robot who robbed his store, by saying that "he was huge, tall, and carried a really long sword". Upon that last phrase, he swings down a broom towards Amato, who catches it between his hands, and an unsheathing sound can be heard.
- The Dreaded: Subverted. Sumorai is seen as the most dangerous of the robotic prisoners. With a reputation of having destroyed a whole military base along with its battalion of robot soldiers before being captured and held in the lowest level of the space prison, all the bad robots fear him. In truth, he was reprogrammed for domestic use after work as a military robot had diminished, and he destroyed the base on accident in a cooking mishap. His scary actions throughout are to protect Oyen-san, his Loyal Animal Companion, including keeping the facade of being intimidating.
- Foreshadowing: Mr. Gobi points Amato to the broken wall of his store that Sumorai broke through after robbing him. An attentive look at the scattered aisle shows cat products on the shelves. It's later revealed that Sumorai is taking care of a cat, but he got startled by Mr. Gobi's panicking and crashed through the wall, a clue to his not so violent nature.
- Samurai in Ninja Town: A variant. Mechamato makes himself a ninja-esque form to battle Sumorai.
- Screw This, I'm Outta Here: Rocketfish is about to punch Amato and MechaBot, but suddenly quivers in fear upon seeing Sumorai right above him on the bridge. Then he begs for Amato to capture him just so he can escape.
