Clammadon slept for centuries deep under the ground of Tokyo, Japan. Above his sleeping place the Japanse build a sumo wrestling stadium.
When Gus along with Tommy and Lola visit Japan to stop Robotboy from destroying fake monsters (he thought that all the monsters were real and so he thought that he was saving the city), Gus decides to go Sumo wrestling instead.
When he and his opponent are stomping on the ground, they accidently wake up Clammadon. Angry that he is waken up, he destroys the stadium and later the city. He also eats Gus his sumo trophy.
Clammadon destroys the city, but Robotboy doesn't stop him because juft before he showed up, Tommy explained that monsters don't excist.
But when Clammadon eats Gus, Robotboy superactivated and tries to stop Clammadon. But Clammadon uses his radioactivity to stop Robotboy and he is brought back to his deactive mode. But then unexpectedly he is being killed by Gus, who destroys him from the inside.
Robotboy throws Clammadon's dead body in to a river and boils the water and creates mussel soup.