Ian Wyndham (Nicholls) is a British Businessman who lives with his musician girlfriend, Samantha Andrews (Hewitt) in London. Ian takes Sam for granted making Sam feeling vulnerable in the role of the one who loves more. Taking us through a day in Ian and Sam's life, the film opens by showing different events such as Sam getting burnt on a kettle, Ian's watch breaking, Sam getting Coca-Cola spilt on her and Ian being interrupted by Sam during an important meeting at work. (She had mistakenly thought that Ian had forgotten his presentation folder at home, when in fact it was just another similar but less important folder.)
As the day progresses we see Ian in a taxi. He tells the driver (Tom Wilkinson) about how he and Sam are going through difficulties. The driver recommends he should just love her. After Sam's concert that night, Ian tries to dismiss Sam's favorite student. At dinner Sam confronts Ian where her frustration boils over. She gets angry at Ian for his attitude toward her and tells him that she just wants him to love her. Sam storms out of the restaurant in tears, Ian following close behind and asking for her forgiveness. Sam gets in a passing taxi and as Ian tries to get in beside her he notices that the driver is the same man who he traveled with earlier that day. The man gives Ian an ominous smile and as the clock hits 11 PM, Sam closes the door, leaving Ian standing there. As he watches the taxi drive away, Ian makes one last attempt to reconcile with Sam and runs after the taxi as it stops at a traffic light. Before he gets there, the lights change and the taxi begins driving again. As it speeds up, the taxi is stuck violently by another car and comes to a halt on the middle of the street. Ian halts and falls on his knees shock.
At a nearby hospital, Sam is being taken into the ER and Ian is running through the hospital looking for her. He arrives outside the room Sam is in and through a windows sees her wounded and surrounded by medical personnel. Sam looks over to the window towards Ian, a blank expression on her face. Ian begins to cry and before the doctors can help her, Sam succumbs to her injuries and dies. Ian falls to the floor in the hospital in disbelief. We see Sam's friend Lottie (Lucy Davenport) enter a hospital room, where Ian is sitting in a chair. They both begin to cry. Ian goes back to his apartment, finds Sam's notebook and opens it, finding a song she was working on. He falls asleep clutching the notebook close to him. As the next day begins, Ian wakes up with the notebook still held tight. He is shocked to hear a voice behind him tell him not to read a word. He jumps up and screams, only to see Sam standing in his apartment. After the initial confusion, Ian comes to the conclusion that the previous day must have been a dream and continues the morning, happy to have Sam by his side
As the morning progresses however, Sam gets burnt by her hair straighteners in the same place she had been burnt by the kettle in his supposed dream. Able to shake this off as a sheer coincidence, Ian takes no further notice of it. Other events similar to those he already experienced begin happening too, although at different times and in different ways, as Sam gets Coca-Cola spilt on her. This time Ian mentions his dream to her, and she convinces him that it was just a dream, and if his watch remains in working order then it can't possibly be the same day as his dream. Nonetheless, Ian is still cautious, and while in a meeting at work he holds his folder in his hands. Sam sees this from outside the glass doors and his meeting is left uninterrupted. Later in the day, Ian gets into a taxi. After talking to the taxi driver he realizes that the driver is the man from the previous day. He questions the man about this and the driver says he does not remember, although he once again gives Ian an ominous smile, implying he may in fact know about Sam's accident and all the previous events. This convinces Ian once and for all, and he runs to Sam, who's helping her friend Lottie set up for an art gallery.
After convincing her to come with him, he brings her to a train station. After much questioning he reveals they are traveling to his rural home town. A delighted Sam makes sure that she'll be back in time for her concert, which Ian ensures they will be. Ian brings Sam up a mountain, to a spot he used to visit as a child. The two find a little abandoned cottage on the mountainside to use as shelter during a rain storm. Upon setting up a fire while Sam sits down, Ian notices that his watch is cracked and isn't working. He turns to look at Sam, who's lighting candles on a table. Sam smiles and when Ian asks what she would do if she didn't have a lot of time left, she replies that she'd do what she's doing right now; spending time with him, just being together. Ian and Sam begin kissing, before making love. Afterwards, they make their way back down the mountain, disappointed that they never made it to Ian's spot. In the town below, the two have drinks and Ian tells Sam about his father, who lost his beloved job in 1993 before becoming an alcoholic and dying sometime later. Ian reveals that he wishes he could have helped his father more, but Sam says he was only a child at the time and wherever his father is now, he's proud of him.
The two travel back to London and Ian takes Sam on the London Eye as another surprise. They then travel back to their apartment and while Sam is distracted, Ian takes a page from her notebook and brings it to a nearby photocopying shop while Sam travels to her concert with her violin. Before the show begins Ian sends a bouquet of flowers up to Sam and gives the photocopied pages to an organizer. A puzzled Sam looks out to Ian in the crowd when her bouquet arrives. His plan is soon revealed however when Ian comes onto center-stage with a microphone. He calls a reluctant Sam to his side, who gets nervous. Ian goes and talks to Sam at her seat and tells her she doesn't want to disappoint her future fans. Sam proceeds onto the stage as the orchestra begins to play the song printed on Ian's photocopied sheets. She sings the song she wrote for Ian in her notebook and the crowd burst into applause at her performance. While walking to a restaurant of Sam's choice, she describes her feelings to Ian.
In an intimate surrounding, Sam and Ian discuss the day in a sheer contrast to Ian's experience the previous night. Ian gives Samantha a charm bracelet with different charms and symbols: a musical note, a violin, a flower (which he describes as exquisite, much like Sam herself), the train they took that day and the Eiffel Tower, which Sam had always wanted to see. The two sit in silence as Sam tears up and smiles across at Ian. As they leave the restaurant in a downpour, Sam tries to get a taxi to take them home. Ian realizes they are in the same spot as the previous night when the accident took place and tells her to look at him and listen. In the rain, Sam listens as Ian professes his love for her and tells her that if it weren't for her or today, he would never have known true love at all. Sam begins crying and says she doesn't know what to say, to which he replies that she doesn't need to say anything.
A taxi pulls up and Sam gets in, beckoning Ian to get in too. He gets in next to her. As the taxi approaches the traffic lights, Ian and Sam kiss. He looks forward and sees the clock strike 11 PM and the same ominous smile he has seen so many times already. Sam smiles at him as the taxi pulls away and images of Sam's accident flash through his mind. He sees a car light through his window and grabs Sam tight for the last time. Sam's scream can be heard. At the hospital, we see Sam's friend Lottie run through the halls. She turns a corner and enters a hospital room, where Sam is sitting in a bed. Sam tells Lottie about Ian's premonition and how she didn't believe him. She and Lottie begin to cry. In the end it was Ian who was the victim of the accident.
Six months later, Sam is sitting in the apartment, with Ian's watch in her hands. Everything is packed into boxes and Sam takes a last look at the room. We see her singing on stage in a restaurant, with Lori at the closest table to her. She travels to the cottage on the mountain which she and Ian visited and continues climbing until she reaches Ian's spot at the peak of the mountain. She stands at the top, staring out into a valley, not knowing what lies ahead.
Source: Wikipedia, If Only