NOTE: This spoiler was submitted by Jeremy
Martha (Anna Kendrick) is home at her apartment when her boyfriend bursts in making out with another woman. While they try to rope Martha into a threesome, she decides to leave as her boyfriend says she has no sense of adventure. Martha replies by throwing something at his head.
We meet the hitman, "Mr. Right"/real name is Francis (Sam Rockwell). He was assigned to kill a man by the man's wife. Francis goes to the woman's apartment and instead kills her after having already collected his payment. Outside, a team of agents led by Hopper (Tim Roth) move into the building to get Francis. One by one, Francis picks them all off and kills the agents.
Martha stays with her best friend Sophie (Katie Nehra) until she gets back on her feet. The two go out drinking, but Martha is looking out to see if her boyfriend could be there. She continues to blame herself for him cheating on her while Sophie tries to persuade her otherwise.
Sophie takes Martha to get a pet. Martha picks out a cat that Sophie warns her is ferocious, but Martha doesn't listen and the cat ends up scratching Sophie. Afterward, Martha goes to a supermarket and runs into Francis when she accidentally knocks over a bunch of condoms, all of which Francis catches quickly. Despite coming off a little creepy at first, Martha lets Francis tag along with her, and the two end up spending the day together. However, several hitmen looking for Francis have eyes on him, but Francis is always one step ahead and avoids getting shot.
Two hitmen brothers, Von and Richie Cartigan (James Ransone and Anson Mount), want to employ Francis to kill a specific target of theirs. Both brothers are at odds with each other constantly, and Von really wants Richie out of the picture. Meanwhile, Hopper (who is really assuming another man's identity) still wants Francis dead.
Martha and Francis go out on an actual date at a restaurant. Francis is pulled out repeatedly when one hitman is outside demanding that Francis contribute his services to the hitman's employers, but Francis refuses and fights the man before ultimately stabbing him in the neck. Francis rejoins Martha and casually says he had to kill someone, which she thinks is just a joke.
At home, Martha comments that she doesn't even know Francis's name after a few days of seeing each other. Francis says he hates his name. Martha then asks him how he caught all the condom boxes the other day. Francis demonstrates his skills with knives as he tries to get Martha to catch them. Although she's freaked out at first, she feels a rush of excitement from catching the knives.
Martha joins Francis one afternoon by a bridge. As Martha waits in the car, she sees Francis meeting with a man on the bridge. The man attempts to get Francis to work for him and his employers, but Francis shoots the man in front of Martha. She gets freaked out and realizes Francis wasn't joking about killing anyone.
After Francis drops Martha off at home, she sits in disbelief until Hopper (saying he's from the FBI) arrives at Sophie's to tell the ladies all about Francis and his work, mainly that he kills off the people that employ him because he abides by a moral code of his, which is that killing is wrong (unless some people deserve it), and also that he wears a clown nose during his kills. Hopper warns Martha that Francis is a very dangerous individual.
Hopper comes across Francis's location, but the hitman gets to him first and knocks him out, then drags him into Sophie's apartment. Martha then forgives Francis for the whole hitman thing, and they start making out. However, one of Hopper's guys tells him through his radio that they're coming for Francis. He gets Martha out of there, just as some bad guys, led by another hitman employed by the Cartigans, Shotgun Steve (RZA), start attacking. Francis takes most of them out, but Martha ends up getting kidnapped.
Martha is taken to the Cartigans home, and Francis follows. Von taunts Martha and smacks her a couple times, but she's not scared of him. Francis decides to team up with Steve, and they storm the house. After taking out several hitmen, Francis decides he's done with killing, and he leaves a gun between Von and Richie for them to settle it. The brothers jump for their gun. Von grabs it and kills Richie. Meanwhile, Martha is set free and kills one of the Cartigans' hitmen before coming across Von. She gets his gun and shoots him to death.
Outside, Hopper confronts Francis, but he gets killed by Steve. Francis tells Steve that Hopper was worth a hefty bounty, making Steve a rich man.
In the end, Francis and Martha continue their relationship, but they are still being pursued by bad guys that want Francis dead. While on a date, Francis writes a message to one of the bad guys surveying them from afar. He writes "Where's the girl?" as Martha is not sitting with him. She appears next to the bad guy with a gun aimed at his head, and she pulls the trigger.