Mario x Meggy: Partners In Time

Mario and Meggy were friends even in their infant years, as they always hung out and played together all day, every day in their childhood. But one day, hell breaks lose when an alien race known as the Shroobs invade the Mushroom Kingdom, and the babies are on their own...

Meanwhile, in the present, adult Mario and Meggy are a couple, and Meggy has since moved in with her boyfriend. E. Gadd has a new time machine that he wanted to show off, and Princess Peach along with two of her Toad assistants use the time machine to explore the past. The time machine returns... But Princess Peach and the Toad assistants do not...

It's up to Mario and Meggy to go back in time and save the princess! Who and what will they encouter in their time traveling adventure?

(This is an alternate timeline, and the Meggy in this book is Inkling Meggy, in case you didn't figure it out from the cover.)