#1: Let's Kick Shell!
And so we immediately kick into the arc of the story with the Turtles going after Shredder and Krang at the Technodrome. In the cartoon series, the turtles were always stopping some plan or another to free the Technodrome.
The Technodrome was Krangs super-military base of god-like power that somehow, stupidly, always ended up trapped in a lava floe or Dimension X or underneath a styrofoam boulder. Krang, being the twitchy brain-thing, always wanted it free so he could take over Earth, and the Turtles always managed to stop him and his plans.
Here, they've already made it to the end of the Technodrome (from "TMNT IV: Turtles in Time" on the SNES if you're curious), escalating the plans to a fever-pitch.

When evil spreads across the land, and darkness rises and the monsters roam. When the creatures of the night make beautiful music, and the things that go bump in the night go bump with greater enthusiasm. When the world is in peril and is in need of a hero...

These guys are, sadly, the best the world can hope for. These are the adventures of the heroes of CVRPG. They mean well, they try hard, and occasionally they do the impossible...

They actually do something heroic.