Heres the basic summary: Before Valentine's Day 2006, Kate leaves a note for the next tenant in the mailbox asking him/her to forward her mail to her new apartment. Kate moves to her new apartment and begins her new job. Even though Alex has now moved into the lake house (2004 his time) and gotten Kate's note, Kate is not aware of it because she is in Chicago and has started her new job. At this point in 2006, Kate has not returned to the lake house.
On Valentine's Day 2006, Kate is having lunch with her mother at Daley Plaza. A man is hit by a bus. Kate goes to his aid and tries to save him, but he dies in her arms. At this point she she has no idea who he is and clearly does not recognize him as the man she kissed at her birthday party back in 2004. I know that this has been a sticking point for a lot of people, but think about it. Back in 2004, Kate came home from the hospital to a surprise party that Morgan threw for her after she clearly said she did not want one. To make it worse they were all Morgan's friends and not hers. She was clearly pissed. Yes, she met Alex and kissed him but he was just some guy that she briefly met for the first time. Quite frankly, I have personally met people at parties, chatted with them for quite some time and don't recognize them two weeks later-it does happen- let it go. Back to 2006, Kate goes back to the hospital after the accident, visibly upset that a man just died in her arms after she gave her best to save him. Her supervisor tells her to go someplace where she most feels like herself on her days off if she's to survive the stress of working in an ER - so she goes back to the lake house.
After Valentine's Day 2006, Kate finds Alex's (2004 his time) letter in the mailbox when she goes back to visit the lake house. Kate begins to exchange letters (2006 her time) with Alex (2004 his time).
On Kate's birthday in 2006, she thinks back to her birthday party in 2004 and starts to remember the man that asked her about her favorite book- Persuasion by Jane Austen- the book she had recently asked (2006 her time) Alex to go fetch from the train station in his time 2004 because she left it on the bench and it meant a lot to her because her father gave it to her. It's on her birthday in 2006 that she realizes it was Alex that she met and kissed at her birthday party in 2004.
Kate and Alex make a dinner date at Il Mare the next day in her time, 2006, the next day two years later in his time. Kate is stood up at the restaurant because Alex died on Feb. 14, 2006. Alex is still corresponding with Kate in his present of 2004. At this point he is not aware that he died.
After the dinner date, 2006, Kate writes Alex (2004 his time) and tells him he did not show up. He replies that something must have happened, give him time, they'll try again. Kate writes a final letter to Alex 2006 her time, 2004 his time). In it she tells him that she has to stop writing to him. She needs to live in the present and find someone in the present. She tells him that life is too short and can be over in a matter of seconds. She writes that on Valentine's Day 2006, she was eating lunch at Daley Plaza with her mother when a stranger was hit by a bus and died in her arms and she wondered if he had anyone else in the world that was waiting for him.
During 2006 through 2008 her time, Kate hooks back up with her old boyfriend Morgan and tries to forget about Alex.
During 2004 after Kate stops writing and February 14, 2006, the day he died, Alex puts all of Kate's letter along with a few other things into a box, puts it into the attic and moves out of the lake house. He moves to Chicago with his brother and starts the Visionary Vanguard agency.
On Valentine's Day 2006, Alex is leaving the Visionary Vanguard agency with his brother in the morning. He asks his brother if he wants to do something together that night. Henry tells him it's Valentine's Day and he has a date. At that moment Alex remembers the final letter that Kate send and remembers that she said where she was on Valentine's Day 2006. Alex heads back to the lake house to reread that letter.
On Valentine's Day 2008, Kate and Morgan meet with an architect (Henry) at the Visionary Vangurad agency. As she's leaving Kate sees the picture of the lake house on the wall and asks who made it. Henry says his brother, Alex Wyler. When Kate asks where he is, Henry says he died exactly two years ago on Valentine's Day 2006. Kate remembers that in her final letter to Alex (2006 her time, 2004 his time) she told him she was at Daley Plaza on Valentine's Day 2006. She realizes that he came to see her on that day and it was him that was hit by the car.
She races back to the lake house and leaves a note in the mailbox saying that he didn't show up because it was him was killed on Valentine's Day 2006. She loves him and if he cares about her, he will not go to Daley Plaza to see her. She tells him that he needs to wait two more years to meet her and that he can not find her or talk to her during that time he needs to just wait until Valentine's 2008 and meet her at the lake house that day.
While Kate is at the mailbox in 2008, with the flag up because her note is in the box, Alex is in the attic 2006 reading her last letter focusing on the words, I was in Daley Plaza on February 14, 2006. As Alex runs out of the lake house clearly on his way to Daley Plaza in 2006, Kate in 2008 watches the run flag going down signalling that Alex got her warning note in 2006.
On Valentine's Day 2006, Alex goes to Daley Plaza where he takes one last look at Kate but does not step off the curb.
On Valentine's Day 2008 (both their times) while Kate is at the lake house, Alex shows up because he waited two years like Kate asked in the note.