A tongue-in-cheek fantasy spy thriller. We don't get enough of those.
Mr. Darcy himself, Colin Firth, plays Harry Hart, an agent for "Kingsman", a super-secret private espionage organization. In a strained allegory, the agency is run by "Arthur" (Michael Caine); there's a technical wizard "Merlin" (Mark Strong); Harry's code name is "Galahad". You get the idea.
But I'm getting ahead of myself. Long ago, Harry's life was saved by the noble sacrifice of a fellow agent. He tracks down the agent's son, "Eggsy", who's living a lower-class life with his slutty mom, hanging out with street thugs. Harry recruits Eggsy into a grueling Kingsman "boot camp", where he competes to be the "last man standing" in increasingly dangerous tests of perilous adventure.
Just in time, too. Because a nefarious plot is in progress, masterminded by "Valentine" (Samuel L. Jackon) and his deadly-but-gorgeous assistant "Gazelle" (Sofia Boutella). It involves massive worldwide casualties in service of (here's a twist) eco-nuttiness.
And also: hey, that's Mark Hamill.