Before becoming a mutant giant, Marugori was working out in his brother's lab. His brother Fukegao gives him the ultimate steroid called "Biceps Brachii King". Fukegao then tells him that it will give him the strength he's always wanted. Marugori drinks it and starts to yell out in pain and exponentially grow in size. Now a giant mutant, Marugori picks up his brother and puts him on his shoulder. They head out to D-City with intent to wreak havoc. Upon arrival, Marugori swings his arm which destroys many buildings. His brother is ecstatic with his overwhelming strength and Marugori begins to head to B-City. It's not long before Fukegao notices something sitting on Marugori's shoulder opposite to the one that he is sitting on. Fukegao tells Marugori that someone is on his shoulder and yells at him to kill the person sitting on his shoulder. Marugori swats the wrong shoulder and kills his brother by accident.
Enraged by the death of his brother, Marugori picks up Saitama and angrily throws him at the ground. As Saitama gets up, Marugori jumps onto him and starts releasing a barrage of punches. which leaves a large hole in the ground.
Believing he has won, Marugori states that he feels empty about what he has become and starts mourning for his brother. Saitama agrees that having overwhelming strength is boring. He jumps up at Marugori and punches him in the face, killing him and sending him crashing into B-City.
B-City is destroyed when Marugori crashes down on the city after being punched by Saitama.
- The battle lasted 1 minute and 15 seconds.
- The hole Marugori created by punching Saitama was 100 m wide.