Gary "Eggsy" Unwin, a young man from a working-class background, lives with his mother and abusive stepfather in a London housing estate. His life takes an unexpected turn when he is arrested for stealing a car. Eggsy contacts Harry Hart, an impeccably dressed man who owes a life debt to Eggsy's late father, a former recruit of a secret, independent intelligence agency known as the Kingsman. Hart, codenamed "Galahad," sees potential in the boy and offers him an opportunity to compete for a position within the organization, where agents are named after knights of the Round Table.

Eggsy undergoes a grueling training program alongside other elite candidates, all while Harry investigates the disappearance of a professor and a connection to Richmond Valentine, a tech billionaire with a speech impediment and a philanthropist facade. Valentine reveals a disturbing plan relating to climate change and seeks to recruit various influential figures to his cause. As Eggsy learns the refined skills of a gentleman spy, adapting to gadgets and tailored suits, the Kingsman agents uncover the true scale of Valentine's scheme, which threatens global security and puts the entire Kingsman organization at risk.