Okay, so, I've never watched The Regular Show. I know it exists and I have a passing familiarity with a couple of the characters, but that's as far as my expertise goes.
That said, you absolutely do not need to possess intimate knowledge of the show in order to enjoy Grudgeball: Enter the Chaosphere.
Ball for one
There's some sort of plot going on involving Mordecai, Rigby, and Co. freezing themselves so that they can perform a rescue operation 1000 years in the future, however it's honestly not that important.
The story is presented well enough using comic book style panels during cutscenes but it's merely a backdrop for the Grudgeball insanity.
Grudgeball itself is something of a pleasant surprise. It's this bizarre mix between volleyball, dodgeball, and hot potato.
The idea is to hang on to the ball for as long as possible to build up power, but you have to pass it back and forth between teammates or else control will pass to the other team.
Then you chuck it at someone's face and hope it either hurts them a lot or knocks them out. Naturally, the last team standing wins.
And one for ball
There are just enough things to focus on (between passing, shooting, blocking, and counter-attacking) that matches never get dull.
Characters will move on their own, which means you'll sometimes need to try and time your specials, but I actually kind of like how that forces you to think and plan a little more than you would have otherwise.
The semi customisable teams, characters that level-up the more feats you pull off, and over-the-top special attacks are pretty great, too.
Passing is sometimes problematic due to the game not registering taps consistently, and on a few occasions I've gone to toss the ball only to either do nothing with it or hand it off to a teammate, but those issues are easy to overlook in the middle of a hectic match.
I honestly wasn't expecting much from Grudgeball: Enter the Chaosphere, but it's definitely a nice surprise.