In 1969, Agent K arrested a criminal named Boris the Animal before he could invade earth. In the process, he destroyed the criminal's arm leaving Boris with just one arm and a bitter hatred for K. In the current year, we see Boris the Animal break out of jail with the help of a girlfriend. With her help, Boris the Animal kills most of the guards of the prison and his girlfriend. They get sucked out of the hole created in the jail. The prison is located on the moon, so the jail is pressurized until the hole is ripped into the side of the building. Boris the Animal cries revenge against Agent K for the loss of his arm and his 40 years in prison.
Agent K finds out about the escape and in a cryptic conversation tells J that he wished he'd killed Boris back in 1969. The next day Agent K is missing, but the only person who remembers K ever existing past 1969 is his partner Agent J. In the midst of a current alien invasion, one caused by the changes Boris enacted in the new 1969, J realizes that he must go back in time to save the world. He must save Agent K, who had installed an ArcNet to protect Earth in 1969. As the alien's invade Earth, just before using the time-traveling device, he wonders how he is the only one to remember Agent K. The device's creator speculates that Agent J must have been there with Agent K in 1969. Though confused, Agent J travels back in time intent on saving his partner, saving the world in the current year and destroying Boris the Animal.
This third movie in the Men in Black franchise returns with Will Smith as Agent J, and the current Agent K is again played by the actor Tommy Lee Jones. When Agent J travels back in time to save the world and Agent K, the younger Agent K is played by Josh Brolin. This latest Men in Black movie was released 15 years after the original movie.