It is 2006, and Dr. Kate Forster is leaving suburban Wisconsin to take a job at a busy Chicago hospital. She is reluctant to leave the beautiful house she's been renting. As she goes, Kate leaves a note in the mailbox for the next tenant, asking whomever to forward her mail and pointing out that the paint-embedded paw prints on the walkway leading into the house and the box in the attic were already there when she arrived.Alex Wyler is a talented but frustrated architect. He arrives at the lake house and finds it neglected - and with no signs of paw prints anywhere. After started to restored it, while he is painting the walkway's railings, a stray dog runs through his paint and leaves fresh paw prints right where Kate said they would be.
Baffled, Alex writes her back, pointing out that the house was unoccupied before he came and wondering how she could have known about paw prints that weren't yet there. Kate, who just left the house a week earlier, imagines he is playing some kind of joke on her, and she fires back a curt reply. Just for argument's sake, she asks, what day is it there? "April 14th, 2004," Alex answers. But for Kate, it is April 14, 2006.
Kate and Alex continue their correspondence and confirm that they are living two years apart. In a letter, Kate mentions a book, Persuasion, she accidentally left at a train station in 2004. Alex goes to the station and finds it. Seeing Kate for the first time, Alex keeps the book, deciding he will return it to her in person some day. Alex sends Kate an annotated map of Chicago and invites her to take a walking tour of his favorite places one Saturday. One day in 2004, while leaving work with Mona, a coworker who has a crush on him, Alex's dog runs away. Alex pursues and meets Morgan, Kate's boyfriend at the time, who invites Alex and Mona to Kate's surprise birthday party, not knowing that Alex and Kate have been writing. At the party Alex talks to Kate about the book. They dance and end up kissing, witnessed by Morgan and Mona.
In later correspondence, Kate recriminates Alex for not saying anything: he points out that she would not have known who he was. Determined to bridge the distance between them, they arrange to meet. Alex makes a reservation at Il Mare (Italian for "The Sea"), a restaurant (whose name is an homage to the original Korean motion picture), for a date two years in Alex's future — but only a day away for Kate. When she shows up for their dinner date she waits but Alex fails to appear. Kate is heartbroken. She tells Alex about a day right after she left the lake house; an unusually warm Valentine's Day when she'd spent time with her mother in Daley Plaza and witnessed a traffic accident and held a man who died in her arms. She asks Alex not to contact her again and stops coming to the mailbox for his letters.
Alex decides to quit the lake house and move in with his brother in Chicago, leaving all of Kate's letters packed neatly in a box in the attic. The dog, Jack, runs away as Alex packs — only to appear at the side of Kate's old boyfriend Morgan, just after Alex passes along the house keys, reminding him of Kate’s wishes to one day live on the lake. The 2006 Kate renews her relationship with Morgan, and they live together in her Chicago apartment for over a year. One afternoon, irritated with his inattention and preoccupation with work, she walks into the bedroom, where a hollow-sounding area under a floorboard finally gets her attention. Stepping hard on one end, she pops the board loose, revealing a small package hidden underneath. It is the Jane Austen book, Persuasion, that Alex retrieved for her from the train station. He has left a flower marking a specific passage: "There could have been no two hearts so open, no tastes so similar, no feelings so in unison...." One unusually warm winter day, Valentine’s Day 2006, something clicks in Alex's memory and he takes off for the lake house.
For Kate, it is Valentine's Day 2008, and she and Morgan arrange to meet at an architectural firm to review renovation plans for an old apartment she wants to buy. Kate notices a drawing of the lake house hanging on the conference room wall. The architect explains that it was drawn by his brother, Alex Wyler who was killed in a traffic accident two years ago to the day. Kate quickly realizes why Alex never met her at the restaurant; he was the man who died in her arms in Daley Plaza. She rushes to the lake house, and writes a note for Alex telling him to wait for another two years and come to the lake house, instead. In this note that she first explicitly professes her love to him.
Alex has gone off to find her and sees her at Daley Plaza in 2006. As he seems about to step into the street, he raises his hand and rereads the note from Kate, begging him to wait for her. "I love you", she writes, "and it's taken me all this time to realize it but I love you". Alex wisely decides to remain on the sidewalk, splitting himself off from the original timeline. Kate falls to her knees, clutching onto the mailbox stand, sure she was too late, but then the mailbox flag slowly lowers - Alex has picked up her note. Soon she sees a vehicle arriving beyond the high grass and then a figure walking toward her on the gravel path, and it turns out to be Alex.
