Admiral James T. Kirk finds himself and members of his former crew on a training mission aboard the Enterprise when they come across an old nemesis, Khan; after Kirk realizes that Spock may not be dead, he and his crew steal the decommissioned Enterprise and head for the Genesis planet, where they encounter Klingons who are after the planet's secrets; while returning home, the Enterprise crew encounter a deadly probe and travel back in time to find a way to save Earth from destruction.