An Inanimate, a shapeshifter capable of disgusing itself as inanimate objects, escapes from prison to Earth. But the MIB get some help from Eileen, an intergalactic cop that stirs up some old feelings for Kay.

Jay mistakes a Cerebro-Accelerator for a pair of head phones, making him the smartest man on Earth. However, this will also cause his brain to explode in twenty-two hours. But his advanced brain is MIB's only hope when someone uses time travel to erase the five founders of MIB from existence.