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Will Smith is back as Agent J in Men in Black 3 – with a double dose of Agent K, young and old -- and ET has an exclusive look behind the scenes at one of this summer's biggest sci-fi sequels!

Video: Will Smith Time Travels in 'MiB 3' Trailer

The long-awaited third installment of the Men in Black franchise, in theaters May 25, finds Agents J (Smith) and K (Tommy Lee Jones) feeling a little too old to battle unwieldy aliens day in and day out. But when history is somehow rewritten and K ceases to exist, J must travel back in time to put things right, literally diving right into 1969 to team up with a younger version of K (played by a pitch-perfect Josh Brolin) to save his partner, the agency, and the future of humankind.

Video: Meet the Newest Man in Black, Pitbull!

Once again directed by Barry Sonnenfeld and executive-produced by Steven Spielberg, the 3D Men in Black 3 also stars Emma Thompson, Jemaine Clement and Michael Stuhlbarg.