The Turtles discover the source of the Mouser robots that trashed their old lair: Dr. Baxter Stockman -- a noted technological genius/inventor, who tries to pass off the Mousers as a means to eradicate the city's rat problem.

When April O'Neil wakes up in the Turtle's lair she has major trouble dealing with the reality of four giant talking Turtles and a Zen-master rat. She soon learns of the origin of the Teenage Mutant Ninja Turtles and they realize their common enemy is Baxter Stockman and his army of robotic Mousers.

After Casey Jones makes himself an unwanted guest in the Turtles' lair, Raphael takes him topside for a "breath of fresh air", where they encounter the Foot Tech Ninjas.

The Turtles, Splinter, Casey and April just barely manage to escape from the explosion that destroys the antique store. They hit the road and hide out at Casey's grandmother's old farmhouse, but Leonardo is badly wounded.

The Turtles and Splinter continue to fight their way up through the Foot Headquarters skyscraper, facing down three misshapen Mutant Shredder Clones, a hallway of traps, and the Mystic Ninja.

With Stockman showing up in the middle of the final battle between our heroes and the Shredder, everything is thrown into chaos ... so much so that our heroes must join forces with the Shredder and Hun to defeat Stockman.