The way events were left in Star Trek Into Darkness, Starfleet and the Federation were at a crossroad where its future and longevity depended on how it saw itself. Was it a military force? Was it an organization that was fearful of making contact with other species or one that embraced contact through diplomacy and friendship? The war with the Klingons was on the horizon and one of the key turning points that will shape a future path for the Federation and Starfleet. And while we might want the Federation to succeed and become a force for good, there are some alien species that would like nothing more than to see it fail. In Star Trek 3, the Klingons could be losing the war due to the reverse-engineered Borg technology Starfleet has and thanks to the actions of Pine-Kirk who is as much of a pain to the Klingons as Shatner-Kirk was. The Klingons would like nothing more than to kill Kirk. In steps a temporal alien, perhaps the type we saw in Star Trek: Enterprise, and spotting an opportunity to turn the tide of the war in the Klingon favour, he gives the Klingons a bargaining chip they can use to mess with Pine-Kirk. That bargaining chip is Shatner-Kirk who has been resurrected and transported into the hands of the alternative timeline Klingons. Pine-Kirk is goaded into rescuing him and both Kirk's together find a way to defeat the Klingons. The end of the movie could have an emotional reunion between the prime versions of Kirk & Spock and in turn bringing the last 50 years of Star Trek full circle.
