Seventeen years later, the climate scientist James Arnold is kidnapped by unknown assailants, led by Gazelle, an assassin with bladed prosthetic legs, and her employer, the internet billionaire Richmond Valentine. One of Hart's compatriots, "Lancelot," attempts to stage a rescue, but fails. Valentine, known for his philanthropy, continues to meet with various heads of state and VIPs, some of whom go missing afterwards. He also announces a giveaway of SIM cards, granting free cellular and Internet access.
In London, Eggsy is now an unemployed young adult living with his mother, infant half-sister and an abusive stepfather. Despite being intelligent and capable, he left training for the Royal Marines and lives an aimless life. After being arrested for stealing a car and taking it for a joyride, Eggsy calls the number on the back of the medal. Hart arranges for his release and tells Eggsy about the existence of the Kingsmen, a secret intelligence agency for which both he and Eggsy's late father worked. In the wake of Lancelot's death, the Kingsmen have a vacancy for a new agent, and Eggsy agrees to apply. He joins a group of other candidates, including a woman named Roxy. Training is overseen by "Merlin," a senior Kingsman. The candidates are eliminated one by one until Eggsy and Roxy emerge as the final two. However, Eggsy fails his final test, and Roxy becomes the new Lancelot.
During this time, the Kingsmen have been investigating Valentine in connection with the VIP disappearances. Hart tracks down Arnold and confronts him about Valentine's whereabouts. A chip implanted in Arnold's head explodes, killing him and injuring Hart. Undeterred, Hart poses as a billionaire and dines with Valentine, who explains his views: humanity is akin to a virus, global warming is the Earth's equivalent of a fever, and Valentine intends to do something about the problem before the virus kills the host.
Hart tracks Valentine to an obscure hate group church in Kentucky where Valentine and Gazelle are conducting a test. They broadcast a signal to his SIM cards, causing everyone in the church, including Hart, to become uncontrollably violent. A mass brawl breaks out, with Hart the sole survivor, while Eggsy, Merlin and Arthur—the Kingsmen's leader—watch via video link. Valentine approaches him, and they discuss his plan: to broadcast the signal worldwide, using his satellite network to cause a massive culling of the human race, saving the Earth from further damage. Only a select few that Valentine has deemed worthy of living—those who have sided with him, and thus received the protective microchips in their head—will be spared. Having revealed his plan, Valentine kills Hart.
Eggsy returns to the Kingsmen headquarters, where he discovers that Arthur is one of Valentine’s converts. After avoiding Arthur’s attempt to kill him—killing the group leader as he does so—Eggsy teams up with Roxy and Merlin. Roxy pilots a high-altitude balloon vehicle into the stratosphere, to disrupt Valentine's satellite network with a missile, while Eggsy and Merlin directly assault Valentine’s mountain bunker. Roxy knocks out the satellite, and Eggsy fights his way through Valentine’s security forces, while Merlin detonates the security chips, killing all who were part of Valentine's plan. Eggsy then fights and defeats Gazelle before killing Valentine and saving the world. In a mid-credits scene, Eggsy, now a full Kingsman, reaches out to his mother, offering her a nicer house and a chance to get away from her abusive partner.