Rachel (Piper Perabo) is a young woman living in North London, England who is getting married to Heck (Matthew Goode), a local stockbroker. Rachel is getting ready by trying on her wedding dress while her bossy mother Tess (Celia Imrie) and adorably dazed father Ned (Anthony Head) are overjoyed at their eldest daughter's upcoming marriage, but only make things more complicated for Rachel. Across town, Luce (Lena Headey) (short for 'Lucy'), is introduced as a young woman who manages a flower shop, and happens to be the wedding florist. Luce realizes she has to make a final delivery and drives off to the church where Rachel and Heck's wedding is. At the church, Rachel's younger 12-year-old sister, Henrietta, adds to the hectic scene by asking why the alphabet list for the guest is in that order and an assortment of other thought provoking questions. Complete chaos is close when all of them finally arrive at the church. When Rachel and her father are walking down the aisle, she catches a glimpse of Luce.
At the reception, Heck's best man Coop (Darren Boyd) has eyes for Luce, not knowing she is gay. When Rachel accidentally drops her wedding ring into the punch bowl, she asks Luce to help fish it out before anyone can see, and the two women again make eye contact as if clicking with each other right away.
A day or so later, Rachel goes to the flower shop to thank Luce for the flowers and the help at the wedding and invites her to supper at her flat on Friday night. Rachel does not realize Luce is gay and invites Cooper thinking the two would make a nice couple. Luce and Heck are talking while Rachel fixes the flowers Luce brought when Heck asks Luce if she is married or ever has been or ever will be. Luce then tells him that no, she is not married but might marry one day since the laws have changed. Heck asks what she means and Luce then reveals that she is a lesbian. Rachel is still in the kitchen while Heck and Luce are talking so she is still in the dark about Luce, and Coop has not arrived yet. Heck takes this news in his stride. Coop is undeterred in his good-natured attempts to sleep with her, even though Heck has now told him about Luce.
The following day, Rachel and Heck are grocery shopping when they bump into Luce and her lesbian friend Edie. At one point, Heck asks Edie if she is gay. She replies that she is and she tells Heck that she is not only gay, but ecstatic. Edie then makes it clear that she and Luce are not a couple, but not for Edie's lack of trying. It is here Rachel learns that Luce is a lesbian, or as Heck puts it, "gay as a tennis player". With this new found knowledge, Rachel has a look about her that tells us she will be seeing more of Luce.
With Rachel working all day at her secretarial job, she asks her co-workers about love at first sight and asks if any of them believe in it. Heck continues working long hours at his office job and feels frustrated about not coming home on time to be with Rachel.
Rachel and Luce continue to grow closer when Rachel takes Luce in tow, along with Rachel's parents, to Henrietta's school to watch her put on a presentation to her science class. Both women grow attracted to each other despite the circumstances surrounding them.
As Rachel continues to grow more confused about whether she may in fact be a lesbian in denial, she goes to a local video store and out of the blue, rents a few lesbian porn DVD's, and takes them home to watch. She hides them from Heck and anyone else who might see them laying around.
Some time later, during a public gathering at Hyde Park for a holiday celebration, Heck confides in Rachel's father, Ned, about him and Rachel growing more apart over his long hours, while Rachel confides in her mother, Tess, about her uncertainty with her marriage. The group runs into Luce who is there with her mother to watch the fireworks and festivities. Henrietta is there and Luce takes the time to further bond with her. Henrietta tells Luce that she got her nickname of 'H' by her own parents when her mother, Tess, became pregnant with her after nearly 15 years and both her parents exclaimed 'Jesus H Christ!' when Tess discovered she was pregnant again. So, the nickname stuck. When Henrietta asks Luce why she is not married, Luce, not wanting to expose her sexual orientation to the youngster, replies that she just hasn't found the right person and that her job of managing her flower shop comes first. Henrietta sort of understands, but replies "That's okay. It's not like you're a lesbian or something."
That same night, Rachel and Heck return to their flat where Rachel works up the nerve to call Luce on her mobile phone, but does not have the nerve to say anything. Luce returns the call on her message ID and Heck answers the phone instead. Feeling that Rachel may be bored with his work schedule, he asks Luce if she wouldn't mind taking out Rachel on evenings while he is busy at work.
A few nights later, Rachel and Luce go to a soccer game where Rachel is yelling and screaming at the players. Luce tells her that she sounds shrill. She then teaches her how to yell and project more from the diaphragm. When they are leaving the stadium, Rachel asks Luce what she would like to do and the two go dancing. They go to a local video arcade and dance to a Kelly Marie song to the Dance Dance Revolution. Later, they are walking down a dark street and Rachel asks Luce what the meaning of flowers are. Luce tells her the azalea means "I wish you wealth". Rachel wants to know the meaning of the Lily. Finally Luce tells her it means "I dare you to love me". At the door to Rachel's apartment, when they are saying good bye, Rachel leans in as if to kiss Luce, but backs away when a car passes. The two back away and say good night.
Over the next few days, Rachel continues to deny her growing attraction to Luce in an effort to remain loyal to Heck. For her part, Luce has no wish to break up a committed couple. Unable to deny what she is feeling, Rachel eventually confronts Luce directly about at the flower shop. She tells Luce that a romance between them cannot happen, and that her marriage to Heck is her sole goal to keep things in her life in line. Rachel leaves, and abruptly returns seconds later to passionately kiss Luce in the back of the shop. Their foreplay gets interrupted by Heck, who has stopped by to pick up flowers for Rachel. While Rachel hides in the back room, she overhears him talking to Luce where he tells her that he has sensed the distance growing between him and Rachel and partially blames himself for not being there for her. Hearing this, Rachel runs off. When Luce returns to the room, Rachel has gone out the window. Luce runs after her and tells her that she can stop it by telling her to leave. Rachel asks Luce if this is what she wants and Luce tells her that she wants her. The two part as they both know the marriage is the driving force for Rachel. Luce tells Rachel not to forget her, Rachel tells her that she will remember nothing else.
As weeks pass by, Rachel gets more and more guilt ridden about her fascination with Luce, and goes so far as to rashly confess all to a drunken Heck, who feigns sleeping, without saying who her lover is. Heck later turns to Coop for support, and Coop angrily confronts Luce after figuring out it was her Rachel fell in love with. Luce decides that being near Rachel without being with her is too hard. She makes plans to go on an extended trip out of the country, leaving care of her flower shop to her mother.
When Rachel and Heck are celebrating her birthday at her parents' house, Henrietta tells them about Luce's trip. Heck then figures out that it was Luce who Rachel fell in love with after noticing her reaction to the news. Heck does not want to be her second choice or to live a lie so he leaves. Rachel then confesses to her parents that she is in love with a woman and with their help, Rachel tracks down Luce as she takes a taxi to the airport. After getting stuck in traffic and Luce brushing her off on her mobile phone, all seems lost. However, Rachel realises Luce is in the same traffic jam and uses the lessons Luce taught her about yelling loudly to call to her. The two women reunite and kiss each other in the middle of the crowded London street.
In a post credits coda, we see the characters some time later. Heck is on the plane to some far flung destination, all set to write his travel book. Henrietta and her young boyfriend are playing in a playground, and Luce and Rachel are still happy together.