Even though agents J (Will Smith) and K (Tommy Lee Jones) have been protecting the Earth from alien scum for many years, J still does not know much about his gruff partner. However, J soon gets an unexpected chance to find out what makes K tick when an alien criminal called Boris the Animal (Jemaine Clement) escapes, goes back to 1969, and kills K. With the fate of the planet at stake, J goes back in time and teams up with K's younger self (Josh Brolin) to put things right.