While you were on a stroll, you suddenly get swept into a portal now turning into a magical girl and you find a hideout. You walk into the hideout and a blobified female with a triple chin, and a yellow and white button shirt notices you and her face lights up.

Mami: Oh my god, thank you for coming on such short notice. I don't have much time to explain, but basically Walpurgisnat fattened me into a giant blob and I can't use my powers. It's up to you to defeat her and get me back to normal.