It's a winter morning in 2006, and Dr. Kate Forster is leaving suburban Wisconsin, where Kate completed her residency, as she prepares to take a job at a busy Chicago hospital. She is reluctant to leave behind the refuge of the woods and the beautiful house she's been renting, an artfully designed home with glass walls that overlook a placid lake.
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-imbedded paw prints on the walkway leading into the house were already there when she arrived.
Alex Wyler is a talented but frustrated architect supervising the construction of cookie-cutter condos at a nearby site. He arrives at the lake house and finds it neglected - and with no signs of paw prints anywhere. The house has special meaning for Alex, having been built by his estranged father, a celebrated architect who let his career grow at the expense of his family, and himself. Like Kate, Alex feels a sense of peace at the lake house and commits to restoring it. He doesn't think twice about Kate's note until days later when, as he paints 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? "February 14th, 2004," Alex answers. But for Kate, it's February 14th, 2006. The same day, two years apart.
As Kate and Alex continue their correspondence through the lake house's mystical mailbox, they confirm that they are, strange as it may seem, living two years apart, and each at a time in their lives when they're struggling to make a new start. Sharing this unusual bond, they reveal more of themselves to one another with each passing week.
In one of her letters, Kate mentions a Jane Austen book, Persuasion, she had accidentally left at a train station in 2004. Alex goes to the station and finds it there on a bench. Seeing Kate for the first time as she boards the train, Alex keeps the book, deciding he will return it to her in person some day. Alex then sends Kate an annotated map of Chicago and invites her to take a walking tour of his favorite places one Saturday morning. Somewhere near the end of the journey, Kate finds a message sprayed as graffiti on a wall: "Kate, I am here with you. Thank you for a lovely Saturday together."
One day in 2004 while leaving work Alex's dog runs away. Alex pursues and meets Morgan who invites Alex to Kate's surprise birthday party. At the party he talks to Kate about the book and she summarises it as being about two people who wait for each other. They dance and end up kissing which is witnessed by Morgan. In later correspondence Kate recriminates Alex for not saying anything but Alex rightly points out that she would not have known who he was.
Determined to bridge the distance between them at last and unravel the mystery behind their extraordinary connection, they tempt fate by arranging to meet. Alex makes a reservation at Il Mare (Italian for "The Sea"), an elegant 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 full of wonderful expectations for their dinner date, however, she waits- but Alex fails to appear.
Kate is heartbroken, and she begins to wonder if she has been making a mistake focusing so much of her emotional energy on a man who, in her time, had clearly moved on. She tells Alex about a day right after she left the lake house, an unusually warm Valentine's Day when she'd witnessed a terrible traffic accident and held a man who died in her arms. Life was too short, she now knew, to wait for what might be. She asks Alex not to contact her again, to "Let me let you go", and stops coming to the mailbox for his letters.
Alex decides to quit the lakehouse 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) 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...." Kate holds the book to her heart.
One unusually warm winter day, Alex and his brother leave their office, heading out to lunch. When Alex suggests they meet up after work for a beer, Henry reminds him that it's Valentine's Day and he has plans with his girlfriend. Valentine’s Day 2006...something clicks in Alex's memory and he takes off for the lake house.
For Kate, it’s 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. Morgan, unenthused about both the project and the idea of moving, has been so busy with work, he has forgotten to get Kate a card. After they meet with the architect, Kate notices an illustration hanging on the conference room wall -it's the lake house. The young man explains that it was drawn by his brother Alex Wyler, who by coincidence 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 killed the day they first began their correspondence - the day she sought solace at the lake house after witnessing a horrible traffic accident.
She rushes to the lake house, leaving a bewildered Morgan behind, and frantically writes a note for Alex. Don't go looking for her, she begs him. Wait for another two years and come to the lake house, instead. It is in this very note, in fact, that she first explicitly professes her love to him. She puts the note into the mailbox and raises the flag.
But Alex has gone off to find her - and sees her sitting there in Daley Plaza on that unseasonably mild Valentine's Day afternoon 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 its 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 kneels by the mailbox, 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. "You waited!", she cries as they begin to make out with each other. And then they turn and, still huddled together, proceed up the wooden walkway toward the waiting lakehouse.
“There could have been no two hearts so open, no tastes so similar, no feelings so in unison....”