You may have heard of the show Gravity Falls, a cartoon that airs on the Disney Channel. Although the show is hit or miss sometimes, its newest game is decidedly a hit for beat 'em up fans.
If you've never seen the show before, that shouldn't stop you from enjoying the game. You may miss a few minor references here and there, but all you have to do is watch the intro and you're good to go.
Honestly, I'd pay $10 for it on the XBLA/PSN.
Rumble's Revenge [Disney]