Hi Hi Puffy AmiYumi S03 E01 Secret Origin

In their childhood years, Ami and Yumi arrive at Camp Youwannasushi and are met by Atchan, who tells them about Rumaki the bunk monster. Yumi is grabbed by Rumaki and Ami and Atchan go under the bed to save her. They manage to save Yumi, but she does not believe them because she was in the sack for the whole episode.