After Alex doesn't show for their date at the exclusive restaurant 2 years into the future (for Alex), Kate gets cold feet and calls the whole thing off. She tells Alex not to write anymore - he does of course, but she doesn't check the mailbox and his letters are not opened.
Morgan the ex-boyfriend (poor guy) is back on the scene, and meets with Kate for a drink after work. He's back working in the city, and soon their relationship is on again. He moves into her small apartment at 1620 Racine (no one has mentioned this ... but isn't that the address of Malone in The Untouchables?? The one written on the back of the matches?)
He likes to take his work home with him - so obviously this alone makes him a bad guy in this movie. Kate is bored, and one day notices a loose floorboard - with the 'Persuasion' book from before hidden for her to find.
Meanwhile Alex is working in the city with his brother, but he still loves Kate. He has decided to sell the Lake House to try and move on. He collects the letters and puts them in a box in the attic, and then gives the keys to Morgan.
On Valentine's Day, Kate and Morgan go to a small architect firm downtown to discuss plans on a dream home - and meet Alex's brother, Henry. As she is about to leave, she spots a plan of the Lake House on the wall. She asks who drew it, and after finding out that it was Alex, she asks where she can find him. Sadly, Henry says that his brother died 2 years ago that day -- Kate realises that it was Alex who was knocked over crossing the street. It is a little strange how she didn't recognise him from the party, but anyway we'll blame post traumatic stress - or she just has a bad memory for faces.
She rushes out the building to stop Alex and get back to the Lake House.
Meanwhile, in the past, Alex is leaving the office with his brother and suddenly realises it is Valentine's Day - and he will know where Kate will be as she has a bad day. To double-check though, of course, and not heading straight to the hospital where he knows she works, he for some reason races back out of town all the way to the Lake House -- gets up into the attic, and searches through the old letters.
Kate arrives. She has somehow (while driving??) had time to write a long and romantic letter, begging Alex not to go into town as it's he who is the person that is knocked over, and will die. She says that she has finally realised she loves him at that moment - so not to bug her for the next 2 years, but to meet her at the Lake House at that very moment. Poor Morgan!
She isn't sure if the letter was read.
Alex races back to the hospital and can see Kate on the bench outside. He is about to wave and step out into the road when the truck approaches ... and passes by him. He read the letter. He decides to step back and wait.
Back in the future, Kate is in tears at the mailbox still when she hears the sound of a truck pull up. Out steps Alex. They run to each other and kiss passionately. Everyone's happy .... except Morgan. And the dog.