Emily Blunt’s character is amoral and manipulative but also rather naïve, so much so that when she sells bad guy Rupert Everett a fake Rembrandt for £900,000, she doesn’t anticipate his getting upset with her.
Billy Nighy is an elite professional killer, possibly the best in the world, who leads a solitary life and is unknown, except by reputation, even to those who hire him. That includes Everett, who contracts with him to kill Blunt.
Nighy fully intends to carry out the contract, but as he stalks her he observes her casual, cheerful, almost non-stop thievery, not to mention her attractiveness, and he becomes fascinated. When someone else tries to kill her, Nighy impulsively becomes her protector.
He acquires an apprentice as well, innocent bystander Rupert Grint (Ron Weasley in the Harry Potter films), who comes to his aid by accident. The three of them end up hiding out together from the vengeance-seeking Everett, who now wants them all dead and has hired Britain’s second-best hit man, Martin Freeman (Bilbo in The Hobbit, Watson in Sherlock).
This is a very, very dark comedy about people who are, excepting Grint, varying degrees of evil, but it’s also frequently quite funny.