Of course, it has an extensive Wikipedia page, written with dead-serious earnestness.

The player guides Mappy the police mouse through the mansion of the cats called Meowkies (Mewkies in Japan) to retrieve stolen goods. 

Oh but there's more.

As part of Bandai Namco's ShiftyLook initiative, an animated Mappy web series, titled Mappy: The Beat, was announced in 2012. The series was written and directed by Scott Kurtz and Kris Straub, who also performed all of the character voices, and presented by using limited animation in the style of their Blamimations series. The story follows Mappy, now working a thankless job as a security guard for his former foe Goro's company Nyamco. He is accompanied by his friends and fellow Nyamco employees, the laid-back digging-obsessed Dig Dug.

There's a video of gameplay as well. Of course.