One: A girl in the family

Mario and Luigi aren't the only children that Mama and Papa have. On a dark and stormy night, the couple finds a beautiful baby girl alone on the streets, known only as Froggy, raising her alongside Mario and Luigi. However, things get shaky when Froggy follows a strange caterpillar whom she befriended, and falls down a pipe, the brothers must venture into many kingdoms to rescue their little sister. However, things are not as they seem.