The film begins with a voiceover from David (Hugh Grant) commenting that whenever he gets gloomy about the state of the world he thinks of the arrivals terminal at Heathrow Airport, and the pure uncomplicated love felt as friends and families welcome their arriving loved ones. David's voiceover also relates that all the messages left by the people who died on the 9/11 planes were messages of love and not hate. The film then tells the 'love stories' of many people:
Billy Mack and Joe
With the help of his longtime manager Joe (Gregor Fisher), aging rock and roll legend Billy Mack (Bill Nighy) records a Christmas variation of The Troggs' classic hit "Love Is All Around". Although he thinks the record is terrible, Mack promotes the release in the hope it will become the Christmas number one single. The song does go to number one; after briefly celebrating his victory at a party hosted by Sir Elton John, Billy recognizes that Joe is the love of his life and suggests that he and Joe celebrate Christmas by getting drunk and watching porn.
Juliet, Peter and Mark
Juliet (Keira Knightley) and Peter (Chiwetel Ejiofor) are wed in a lovely ceremony orchestrated and videotaped by Mark (Andrew Lincoln), Peter's best friend and best man. The video Mark recorded reveals that he is secretly in love with Juliet, which she realizes when she barges into his studio and watches it. Juliet thought he didn't like her because he never talked to her, avoided her, and generally behaved distant and uncomfortable towards her. He blurts out that he acts that way because of 'self-preservation', and she finally realizes that he's always been head over heels for her, but has kept it to himself out of respect for her and Peter's relationship. Mark takes a moment to truly confess his love for Juliet, and Juliet acknowledges him by giving him a kiss.
Jamie and Aurélia
Writer Jamie (Colin Firth) first appears preparing to attend Juliet and Peter's wedding. His girlfriend (Sienna Guillory) misses the ceremony to sleep with his brother. Crushed by this, Jamie retires to his French cottage where he meets Portuguese housekeeper Aurélia (Lúcia Moniz), who speaks only her native tongue. There is an instant attraction between the two. When Jamie returns to England he realises he is in love with Aurélia. Jamie spends time learning Portuguese and goes back to propose to her. She says yes, and it is revealed that she has been learning English to communicate with Jamie.
Harry, Karen and Mia
Harry (Alan Rickman) is the managing director of a design agency; Mia (Heike Makatsch) is his new secretary. For Christmas he buys her an expensive necklace from jewellery salesman Rufus (Rowan Atkinson), who elaborately wraps it while Harry becomes increasingly nervous in fear of detection. Meanwhile, Harry's wife Karen (Emma Thompson) is busy dealing with their children, Daisy (Lulu Popplewell) and Bernard (William Wadham), who are appearing in the school Nativity play. She voices her fears about Harry to her brother David (Hugh Grant), who just became Prime Minister of the United Kingdom, and her friend Daniel (Liam Neeson), who has just lost his wife. Karen discovers the necklace in Harry's coat pocket and initially assumes it is a gift for her; she later confronts Harry over the necklace and he admits his foolishness.
David and Natalie
Karen's brother is the recently elected British Prime Minister David (Hugh Grant). Natalie (Martine McCutcheon) is a new junior member of the household staff at 10 Downing Street and regularly serves his tea and biscuits. Something seems to click between them. David walks in to find the U.S. President (Billy Bob Thornton) attempting to seduce Natalie, which inspires him to take a stand against the bullying President's policies. Finding that his relationship with Natalie has become strained, David has her moved to another job, but later comes across a Christmas card from Natalie ending with a declaration of her love for him. He eventually finds Natalie at her family's home (next door to Mia), seeing that they are on the way to the local school for the nativity play (the same one in which his niece and nephew are appearing). He drives her family to the play, and the two watch the show from backstage, their budding relationship exposed when a curtain is raised on them kissing at the end of the pageant's big finale.
Daniel, Sam, Joanna and Carol
Daniel (Liam Neeson), Karen's friend, and his stepson Sam (Thomas Sangster) fend for themselves as they mourn the loss of their wife and mother. Sam has fallen for American classmate Joanna (Olivia Olson), and, after discussion with his stepfather, decides to learn the drums so that he can accompany her in the big finale for their school's Christmas pageant (the same one that Natalie's nephew and Karen and Harry's children are in). After Sam feels that he missed his chance to make an impression on her, Daniel convinces Sam that he must go catch Joanna, who is returning to the US, at the airport that night and show her how he feels, lest he regret it for his whole life. Sam runs away from the airport security and says hi to Joanna, who then kisses him on the cheek. Meanwhile, Daniel, who famously lusts after supermodel Claudia Schiffer, meets the parent of another of Sam's school mates, Carol (Claudia Schiffer), who is a dead-ringer of Ms. Schiffer.
Sarah, Karl and Michael
Sarah (Laura Linney) first appears at Juliet and Peter's wedding, sitting next to her friend Jamie. We learn she works at Harry's graphic design company, where she has been in love for years with the company's creative director, Karl (Rodrigo Santoro). A tryst between Karl and herself is interrupted by Sarah's mentally ill brother, Michael (Michael Fitzgerald), and this effectively ends their relationship. On Christmas Eve, she visits her brother at the institution where he lives, wrapping a scarf around him as he hugs her.
Colin, Tony, Stacey, Jeannie, Carol-Anne, Harriet and Carla
After several blunders attempting to woo various English women, including Nancy (Julia Davis), the caterer at Juliet and Peter's wedding, as well as Mia, Colin Frissell (Kris Marshall) informs his friend Tony (Abdul Salis) he plans to go to the US and find love there, convinced that his Britishness will be an asset to him in a foreign country. Landing in Milwaukee, Wisconsin, Colin immediately meets Stacey (Ivana Milievi), Jeannie (January Jones), and Carol-Anne (Elisha Cuthbert), three stunningly attractive women who fall for his Basildon accent and invite him to stay at their home, where they are joined by roommate Harriet (Shannon Elizabeth).
John and Judy
John (Martin Freeman) and Judy (Joanna Page) meet as body doubles for the sex scenes in a film where Tony is a production assistant. John tells Judy that "it is nice to have someone [he] can just chat to." While the two are perfectly comfortable being naked and simulating sex on-set, they are shy and tentative off-set; they carefully pursue a relationship, and see the play (same one as David & Natalie, Harry & Karen, Daniel & Sam, et. al.) at the local school together with John's brother.
Rufus
Rufus is a minor, but significant, character played by Rowan Atkinson, the jewellery salesman whose obsessive attention to gift-wrapping nearly gets Harry caught by Karen buying Mia's necklace. Also, it is his distraction of staff at the airport which allows Sam to sneak through to see Joanna. In the director and cast commentary, it is revealed that Rufus was originally supposed to be a Christmas Angel, however it was dropped from the final script.
Epilogue
One month later, progress is shown in each relationship: love, actually. However, two of the relationships are not romantic - those of rock star Billy Mack and his manager Joe and of long-suffering Sarah and her institutionalized brother Michael. Billy arrives with a gorgeous groupie in tow to be greeted by Joe and indicating that his comeback has remained successful. Juliet, Peter, and Mark come to meet Jamie and his new bride Aurelia, showing both that they have married and that Mark is learning to cope better with the romantic triangle. Karen and the kids greet a returning Harry confirming that they have survived their relationship crisis and reconciled. Sam greets Joanna who has returned with her mother from the US as Daniel looks on, joined by his new girlfriend Carol and her son. John and Judy run into Tony, who is there to meet Colin on his return from the US; they are shown to be just married and off to their honeymoon. Colin returns with the gorgeous Harriet in tow, and Harriet's younger sister Carla (Denise Richards) and Tony are immediately struck by each other. Natalie is seen welcoming David back in front of press cameras, indicating that their relationship is now public. These scenes dissolve into live-footage of actual arrivals at Heathrow Airport, which divide the screen and eventually form a heart as The Beach Boys' "God Only Knows" plays on.