TMNT and Splinter watching on TV a story about Dr. Baxter Stockman and his new invention the rodent-killing robotic Mouser demonstrated by his lovely assistant April O'Neil.
Several weeks after, there are fewer rats and an increase in bank heists. April talks to Baxter about how the Mousers could have done the bank heist easily. Baxter assures her that no one has taken control on the Mousers and had them do things that they were not programed to do. Baxter then takes April down to the subbasement and shows her an army of Mousers and tells her that it was he who stole from the banks to get enough money to build an army. April runs from Baxter but Baxter had lead her into a trap. There was one escape for her; the storm sewers. Baxter sent three Mousers after her.
The Turtles destroy the Mousers and April faints at the sight of the Turtles. April later wakes up in their home. Splinter tells her their origin story.
Meanwhile, Baxter threatens to demolish buildings unless he is given ransom money. April agrees to lead the Turtles to Baxter's underground lab.
The Turtles easily fight Baxter but not before he programs the Mousers to destroy everything at the Turtles current location. Donatello and April shut down the mousers while the others were fighting them off.