It was first heard when Natsumi is pulled out from Meru's body as Meru's body glow orange and becoming more shapeless blob. He screams in pain and for help, so Maru came as she sucked into Meru's body. Inside, Meru and Maru are inside as Meru changes back into his original form and Kururu mentions that Keroro and co. should bring them out before the underwater city will explode. Natsumi accepts especially Keroro to rescue the two. After the events, they succeeded for rescuing Meru and Maru.