Leaving out some lore, Bad guys have invaded the sewer and frozen everything before pumping in toxic waste. Casey (and the turtles) are there to take a stand and drive them back, giving them a taste of their own medicine. Biggest overview? Turtle themed Kneecapper practice.

Hit guys into goals (pipes) to score for your team. Hire turtles, April, Casey to fight with you (while pathing poorly). Buy AOE heals to try to keep hirelings alive. Knocking guys into Shredder or the fan do extra damage. 3 periods, with intermissions.
Intermission 1 has a "defeat Mousers, give Don scraps" quest to stop them spawning (and give you another bat). Intermission 2 the actual Kang Android (and not the malfunctioning other ones with the short-circuit aura) shows up, and drops Bass Boosts. Direct (non goo) elims grant you Goongala skill points, which cap out at level 3 for now.

Dev goals eventually: Change scoring to be less "point for Assist" and more "point for team 1 if someone is eliminated by team2 goal" allowing for self-scoring on accident, and clearer gameplay. But devices don't support what I need for now. More cinematics. Opening convo with April and Casey. Persistence, unlocks, retention.

I might eventually make a Casey HomeBase Hub and this is just one of the things you can do off of it.