This game happens some time between SMB3 and SMW, probably.
Peach has just been rescued by Mario, and Mario reminds her that she still owes him a cake, so she goes to make one. However, she realizes that her super secret Cake(TM) Formula had been stolen! She's not sure who did it, but she's certain that it must be Bowser. Once Mario comes to get the cake, she tells him this, and Mario is frustrated. So, off he goes to get the formula back!
There's more than this, but the demo does not currently have it yet.
