The USS Enterprise embarks on a critical mission to observe the inhabitants of Planet Nibiru, a pre-warp civilization teetering on the brink of destruction. Captain James T. Kirk (Chris Pine) and Spock (Zachary Quinto) race against time to avert a catastrophic volcanic eruption that threatens to obliterate the populace. When Spock's life hangs in the balance, Kirk makes the drastic decision to breach the Prime Directive, exposing the Enterprise to the civilization as he risks everything to save his friend. Their actions garner the reverence of the indigenous people, who begin to see the ship as a symbol of hope as it departs.
Upon returning to Earth, Kirk faces the harsh consequences of his choice, demoted to First Officer and replaced by Admiral Christopher Pike. Meanwhile, a sudden attack by John Harrison (Benedict Cumberbatch) leads to a devastating bomb explosion at a covert Starfleet installation. Following this, Pike attends an emergency meeting at Starfleet headquarters, where Harrison strikes again, ultimately killing Pike. In the aftermath, Kirk retaliates and destroys the gunship, but Harrison escapes into the depths of Klingon space.
With Pike gone, Admiral Alexander Marcus commands Kirk to pursue Harrison, who has sought refuge on the Klingon home world of Kronos. Given the imminent threat of war with the Klingon Empire, the Enterprise is equipped with 72 advanced prototype photon torpedoes and ordered to eliminate Harris once he is located. However, in protest against the potential dangers posed by the torpedoes, Montgomery "Scotty" Scott resigns, which prompts Pavel Chekov to step up as Chief Engineer. Under an assumed identity, Carol Marcus joins the mission, adding another layer of intrigue to the crew.
As the Enterprise navigates towards Kronos, a malfunction in the warp core impedes their progress. With repairs underway, Kirk, Spock, and Uhura take a commandeered trader ship to meet their fate on Klingon soil. When Klingon patrol ships detect them, their diplomatic efforts fail, leading to a tense standoff. Harrison, showcasing extraordinary strength, intervenes and quickly incapacitates the Klingon forces, but chooses to surrender when cornered by Kirk’s team.
Upon revealing his true identity as Khan Noonien Singh, a genetically enhanced superhuman averse to his past, Harrison warns Kirk about the torpedoes, which, he claims, harbor his cryogenically suspended comrades. Admiral Marcus had awakened Khan, leveraging his remarkable abilities to forge cutting-edge weapons for impending warfare while holding his team hostage. Realizing that his foes had sabotaged the Enterprise under the Admiral’s orders, Kirk rallies his crew to face the impending danger.
After Scotty infiltrates a clandestine Starfleet shipyard to repair the Enterprise’s warp core, they cross paths with the dreadnought USS Vengeance, commanded by Marcus himself. Seeking the blood of Khan to harness his superhuman power, the confrontation escalates as Kirk remains steadfast against surrender.
As the battle ensues, clever tactics unravel; amid chaos aboard, Kirk offers a deceptive truce to save his crew. Despite Khan's betrayal leading to the death of Admiral Marcus, the Enterprise crew discovers a way to turn the tides with the clever distribution of their remaining torpedoes. The conflict comes to a head as both ships plunge towards Earth’s atmosphere.
Kirk’s sacrifice ensures the crew regains control of the ship just in time, leading to a dramatic outcome where the Vengeance collides in downtown San Francisco. In the chaos, Khan attempts a desperate escape but is thwarted by Spock, while McCoy uncovers a possible remedy in Khan's blood that could bring Kirk back from the brink of death.
In the denouement, Kirk is revived and reinstated as Captain of the Enterprise, while Khan is sealed away for good as the story closes with the ship being re-christened, setting out for a new five-year exploratory mission across the stars.
