Leonardo is away training and the rest of the gang have gone on to other interests than ninjaing. Splinter becomes afraid that a common enemy, Shredder, was the only thing keeping them together as a family. As Leonardo returns a new threat grows in the city, a wealthy industrialist, working alongside Karai and the Foot, has summoned an army of monsters into the city. The TMNT are forced to reunite and fight this new threat.
Something along those lines...