NOTE: This spoiler was sent in by Renee who says... "It's a very good film, sweet and funny but with a little sadness in it as well.

The film opens in late-1930s London with Guinevere Pettigrew (Frances McDormand), a governess, getting fired by her drunken employer. She trudges off down the street, at one point getting bumped by a young man so that her suitcase flies open and all her things spill out. He apologizes and tries to help her, but when he remarks that he’ll probably be sent back to prison for this, she flees and leaves everything, even though he tries to explain that he was just joking. (Important later.) Miss Pettigrew gets a meal at a soup kitchen. As she carries her food outside, she sees an apparently tipsy couple making out. The woman sees her and tells the man, Jerry, that they should go elsewhere. (Also important later.) As Miss Pettigrew watches them go, someone bumps her from behind and she spills her food. She also spends some time resting at the train station, where she looks at headlines about the coming war.

At the employment office, the director, Miss Holt, says she can’t get any more jobs for Miss Pettigrew because of all the times she’s been fired, for various reasons. Miss Pettigrew claims that "people can change," for instance, her former employer could stop drinking but Miss Holt says it's the governess’s job to adapt to the employer, not the other way around. When Miss Holt has to leave the office for a moment (we hear her telling someone about getting a new social secretary for Carole Lombard, a movie star of the time (important later), Miss Pettigrew sees a card on the table with the name "Delysia Lafosse" on it. She snatches it up. At Delysia's building, Miss Pettigrew nervously waits for her to answer the door, and then tells Delysia that she’s from the employment agency. Delysia (Amy Adams) asks Miss Pettigrew the time and is shocked to hear it’s five after ten. She dashes back inside and frantically starts cleaning up the messy living room, responding gratefully when Miss Pettigrew helps her get her bra off the chandelier. She mentions someone named Phil upstairs whom she needs to get out of bed right away. Miss Pettigrew thinks she means her little boy, and volunteers to get him up and dressed.

Miss Pettigrew marches into the bedroom upstairs and orders Phil to get up. When he doesn’t, she yanks off the covers, to discover that Phil (Tom Payne) is a naked man. We see his backside; Miss Pettigrew sees his front. She gets back downstairs, where the phone rings. Delysia instructs her that if it’s someone named Nick, to say that Delysia's dead. But it’s Miss Holt, wanting to tell Delysia that she’s sending over a new social secretary for her. In a German accent, Miss Pettigrew tells her that Delysia is dead, and hangs up. As she starts to tell Delysia what happened upstairs, the phone rings again. It’s Nick, who’s on his way over, which means they have to get Phil out immediately. Delysia explains to Miss Pettigrew on the way upstairs that she’s actually juggling three men: Nick, Phil (a theater producer's son), and someone named Michael.

As they try to get Phil out of bed (and he tries to get Delysia back into bed), it’s explained that Phil is putting on a show, "Pile on the Pepper," in which Delysia wants the lead role, but to please his father, he has to see an actress named Charlotte Warren over lunch about the part. To help get Phil up and out, Miss Pettigrew claims she saw Charlotte with another producer. That does the trick, but once downstairs, he holds everything up by asking for breakfast. As Miss Pettigrew prepares toast, she tells Delysia that she doesn’t have to let Nick in, but she says she does because it’s Nick’s apartment. She sneaks Phil out by the elevator while Nick’s coming up the stairs.

Miss Pettigrew is uncomfortable with the whole situation because morals are important to her, and she wants to leave. Delysia begs her to stay because she needs "saving" from Nick. She explains that with him, she’s like a small furry animal hypnotized by a snake, and she needs someone to be strong for her. Miss Pettigrew agrees to help, but she tells Delysia that she needs to be strong herself.

Nick (Mark Strong) comes in and Delysia pretends to be ecstatic to see him. She introduces Miss Pettigrew as her social secretary. He’s suspicious because he thought he saw Phil’s car in the street and Delysia in the elevator, and then he sees Phil’s cigar in the ashtray. Miss Pettigrew sticks the cigar in her mouth and declares, "If I want to smoke a cigar I’ll damned well smoke a cigar, thank you very much, and to hell with your opinion." They finally get Nick to leave by telling him that Delysia will be late for a fashion show, but he promises to come back for her at night. Miss Pettigrew is upset about what "desperation" made her do, because she’s a vicar’s daughter and has never sworn or smoked before.

As Delysia gets dressed for the fashion show, it comes out that Miss Pettigrew is broke and has no other clothes besides what she has on. Delysia somehow gets it in her head that Miss Pettigrew lost all her money gambling since working for Carole (Miss Pettigrew doesn’t understand what she means by that). Delysia promises to buy her some things after the lingerie show. On the way to the show, she takes off her scarf and gives it to Miss Pettigrew out of gratitude for all her help.

At the show, the models are introduced by a woman named Edythe (Shirley Henderson), whom Miss Pettigrew will come to realize was the woman she saw kissing the man called Jerry near the soup kitchen. Delysia tells Miss Pettigrew that Edythe is dating Joe (Ciarán Hinds), a lingerie designer who is there for the showing of his latest collection. Edythe says a few nasty things about Joe’s collection, from which Delysia deduces that they’ve had another bust-up. After the show, Miss Pettigrew goes to get some food at the buffet table. She meets Joe, who tells her the scarf looks good on her. She says it’s the most beautiful thing she’s ever worn, which pleases him, as it turns out he designed it. As they talk, she drops her food again; he stoops to pick it up and she escapes. She looks at appearance in a mirror and sadly takes off the scarf. Meanwhile, Edythe is telling Delysia that Joe suspects her of being with Jerry, but she claims she’s really innocent. Delysia invites her shopping with them. It comes out that Delysia thinks that Miss Pettigrew used to work for Carole Lombard.

Miss Pettigrew gets a facial while the other women pick out clothes for her. With new clothes and a new hairdo, she looks like a different woman. While Delysia is gone for a few minutes, Edythe tells Miss Pettigrew that she’s recognized her. She wants Miss Pettigrew to keep her mouth shut about Jerry and help her smooth things over with Joe, in return for her not telling Delysia that Miss Pettigrew was really a penniless vagrant.

When Delysia and Miss Pettigrew get home, someone’s there playing the piano. It’s Michael (Lee Pace), Delysia's third boyfriend and the man Miss Pettigrew bumped into near the beginning of the movie. (It comes out that he and Delysia also work together as a pianist and singer at Nick’s nightclub.) He picks Delysia up, shakes her, and sits her down hard on the piano and they have a fight. She didn’t turn up one day when he was planning to propose to her (she says she was scared), so he got so drunk that he tried to steal jewelry for her from the Tower of London and got sent to jail for a month. He tells her that he wants it to be just the two of them from now on, with no more Nick or Phil. He has two tickets on the Queen Mary to New York, sailing the next morning, and he wants her to come with him. During all this he’s very charming to Miss Pettigrew and wins her over to his side, but she argues that he should give Delysia a little time to consider her answer. When he leaves, she pronounces him "magnificent' while Delysia calls him 'impossible."

While taking a bath, Delysia frets. She really loves Michael and knows he loves her. But she really wants the part in Phil’s show too. Miss Pettigrew tells her that there are times when we have to make choices, but she doesn’t want to accept that.

Some people show up with food and flowers, and it turns out Delysia is hosting a party she forgot to tell Miss Pettigrew about. It’s where Phil will announce who got the lead in his show. He shows up at the party with Charlotte, and while she and Delysia snipe at each other, Miss Pettigrew rescues him and takes him to the balcony. Phil confides in her that he’s giving Charlotte the role, and that he doesn’t think Delysia really loves him. To win the role back for her, Miss Pettigrew tells him that she does. He’s elated and goes off. Joe and Edythe are also at the party, though Miss Pettigrew thinks he doesn’t recognize her since her transformation and doesn’t give her name. Edythe keeps trying to make Miss Pettigrew live up to her end of the bargain, but Joe isn’t interested in getting back with Edythe. At one point, Miss Pettigrew, fed up, cries out to him, "You're all playing at life. . . . Love is not a game," and goes off. Meanwhile, Phil wants to make a double announcement: that Delysia will play the part of Hortensia, and that they’re going to be married. Delysia agrees to marry him. When the announcement is made, Michael, who’s been playing the piano for the party, looks deeply hurt. His eyes meet Delysia's for a moment and then he leaves while she dances with Phil. Miss Pettigrew is sitting alone and Joe goes to sit with her. They hear war planes fly over and the other partygoers crowd outside to watch. Miss Pettigrew remarks, "They don't remember the last one," and Joe answers, "No, they don't." Edythe shows up and tells Joe to take her home, which he does. After the party, Miss Pettigrew is ashamed of what she did to bring Delysia and Phil together, because Michael is the one that loves Delysia for who she really is and not who she pretends to be. Although Miss Pettigrew has hinted that she was once in love, Delysia tells her she doesn’t really know anything about it. Miss Pettigrew answers that she knows all about the lack of love and wants to save Delysia from that fate.

Delysia has to work at the nightclub that night. On the way there, she tells Miss Pettigrew that her real name is Sarah Grubb, her father is a Pittsburgh steelworker, and that she’s terrified of being poor again. Miss Pettigrew tells her they’re alike in that way. Delysia says that only Michael knows the truth about her, but he doesn’t judge her.

At the nightclub, Nick knows about Delysia's engagement but isn’t bothered by it because he knows it's just business Edythe and Joe are there, and their engagement is back on, but she wants to dance with someone else, so Joe ends up dancing with Miss Pettigrew. She seems nervous and he asks if he makes her uncomfortable, but she says she’s never been more comfortable in her life. When Delysia comes out to sing, Nick announces an upbeat song, but Michael plays a slow romantic one instead. Delysia starts to cry as she sings it, and he starts to sing too and they finish it together. Miss Pettigrew leads the applause. Then the lights go out and the air-raid sirens go off, and everyone tries to find cover. Miss Pettigrew goes to Delysia, who’s under the piano, and who’s just starting to comprehend that there’s going to be a real war. Miss Pettigrew tells Delysia that the man she loved was killed in the First World War, and life is short so you have to go with what your heart tells you. When the lights come back on, Delysia sees Phil kissing Charlotte Warren, but she doesn’t care any more. She runs after Michael, who’s leaving. Nick goes after her and orders her to go back and sing, but she doesn’t want to. Nick and Michael get into a fistfight (urged on by Miss Pettigrew yelling to Michael to sock Nick in the jaw), in the midst of which Delysia tells Michael that shell marry him if he still wants her. He knocks Nick out and kisses her as everyone applauds.

Miss Pettigrew tries to leave but finds herself in a dead-end alley, where she sits down, and Joe finds her there. While they’re talking about how he originally made men's socks and wants to get back to that, Edythe comes upon them and says that because Miss Pettigrew spilled the beans about her and Jerry, she’s going to spill the beans about how Miss Pettigrew was a vagrant who hung around soup kitchens and train stations. Joe tells her that Miss Pettigrew didn’t tell him anything, but he’s glad to hear the truth from her for a change, and leaves.

Miss Pettigrew gets back to the apartment as Delysia and Michael are leaving for the Queen Mary. Delysia and Miss Pettigrew have an emotional goodbye and swear they’ll never forget the day they spent together.

Miss Pettigrew is back at the train station, looking hungrily at a half-eaten apple someone has left, when Joe finds her. He says he’s been looking for her all his life and wants to marry her. She’s too stunned to speak, so he asks her just to nod, and she happily does. As she takes his arm and they start to leave, he asks if she’s had anything to eat, and she says she hasn’t eaten anything in a very long time. She lays her head on his shoulder as they go out.