Just saw "The Lake House". Can anyone give me a good synopsis and explain the most confusing points ?

For instance: Alex is hit by a bus while going to meet Kate. She rushes to his aid. Later she is in the restaurant waiting and of course he never shows up because he was killed. Yet, they show him writiing that he doesn't know why he didn't show up, "something must have happened". Then she later saves his life by asking him to wait......Can anyone sort out the confusion?