It's been several years since Agent K (TOMMY LEE JONES) and Agent J (WILL SMITH) last worked together in saving the world from extraterrestrial aliens. Working for the top secret Men In Black government operation that's run by Zed (RIP TORN) and is dedicated to policing and monitoring all alien activity on Earth, K decided he'd had enough of the job and wanted J to "neuralize" him, thus erasing all memories of his job and encounters with various aliens.
Since then, J has become more jaded with work, especially when partnered with incompetent agents like T (PATRICK WARBURTON), or annoying ones such as Frank (voice of TIM BLANEY) who's disguised as a vocal pug.
Upon investigating an apparent alien on alien killing observed by human witness Laura Vasquez (ROSARIO DAWSON), J discovers that an evil Kylothian alien by the name of Serleena (LARA FLYNN BOYLE) has returned to Earth in search of the light of Zartha, an energy source that's so powerful that the Zarthians entrusted the Men in Black during the 1970s to keep her from getting her vine-like tentacles on it.
With the aid of her dimwitted, two-headed alien accomplice Charlie (JOHNNY KNOXVILLE), Serleena - now disguised as a lingerie model - is trying to track down the Light. Accordingly, Zed sends J to find and bring K - now Truro, Massachusetts postmaster Kevin Brown - back into the fold, as he's the only one who knows the Light's whereabouts.
Paying a visit to local alien Jeebs (TONY SHALHOUB), J de-neuralizes K and the two then set out to save Earth by finding and stopping Serleena before she acquires the Light.