Chronological order of events
2004
Alex finds the first note, telling him about the footsteps on the bridge
Alex paints the bridge, Jack appears and creates the footsteps
Alex and Kate correspond
Jack runs away, Alex abandons the lake house and gives the key to Morgan, so Kate can move in
2006
Kate leaves the lake house and moves to Chicago, takes Jack with her, but leaves the first note about footsteps and a box in the attic.
On Valentines Day, Kate witnesses a man dying in a traffic accident at Daley Plaza; though a doctor and on the scene within seconds, the man dies "in (her) arms," she later tells her lead doctor
While visiting the lake house on a weekend, she finds a response letter
Alex and Kate correspond
Kate waits at the Il Mare, but Alex does not turn up
She tells him not to write to her anymore
2007
Kate rekindles her relationship with Morgan
2008
On Valentines Day, Kate and Morgan visit a successful renovation contractor who turns out to be Alex's brother, Henry. When Kate notices a scetch of the Lake House, she asks Henry who made it. He tells Kate that Alex did; she asks to speak to him--Henry pauses and says sadly that Alex died two years ago on Valentines Day. Kate asks where it happened
Kate rushes to the Lake House ans its mailbox. She leaves a desperate message in the mailbox, telling Alex not to come to meet her at the Plaza, not cross the street and just wait--she is waiting at the Lake House.
Weeping, Kate almost fails to notice the telltale turning-down of the mailbox "flag". She stands, turns around, Alex appears in his pickup. As she says, "You waited," the movie ends.