Sad and lonely janitor recollects a made-up tale involving himself and a girl he dated or might have made up.
Janitor hangs around high school being sad. TV and the high school play makes him feel a little better.
Janitor has an episode and dies in pickup truck.
The rest is basically a mix of his personal experiences when younger and fiction.

It also addresses the way many people live with a great story in their head that they never get to publish or film. Sad janitor walking around with story he never got to tell.

He never asked the girl out and thought "What if", which was her planning to end it anyway so ultimately he wound up where he always would have, a pity party.

Then he basically speculates that she could have stuck around, and life could have been better.

There's some complaints about the film because Lucy "isn't real." Except, she is real, as real as Jake is. The Janitor is basically thinking both their thoughts. The gag through the movie is that Jake can always hear Lucy's thoughts, and he's often reacting to her thoughts without trying to seem like it.

Oklahoma! is referenced a lot. I've never seen the musical, but I recognized the ballet sequence being a reference to it because of the parody done in Cannibal: The Musical. I gathered the final song Jake sings on stage was another reference, and looking it up, it's "Lonely Room" from that musical. Earlier in the car, the Oklahoma song comes on the radio because the Janitor is, at that point, watching the rehearsal at the high school. As Jake says, the high school puts it on every few years, so The Janitor has seen it a lot. That's also probably why he's seen so many other musicals and can list so many, despite not really being "into" musicals.

"A Robert Zemeckis film" . Later in the film, in the car ride away from the farmhouse, Lucy is very briefly played by the actress in that film. Lucy's whole backstory changes to the one the character in that movie had, and her name changes to "Ames."

The scene at Tulsey Town seems to indicate that the girls there are more permanent fixtures in The Janitor's mind. The dumpster full of cups suggests that The Janitor daydreams this scenario a lot, but he ultimately always ends up a Janitor at the high school, and has to go home. I think The Janitor ended up living in his parent's old home. The opening shots are all of the farmhouse, but later, remnants of his parents still lingering around. I think he's done this loop a lot, with variations of Lucy and other women he thought about approaching, but never did. The swingset that Lucy sees early on is the swingset from the farmhouse, but from earlier in his life.

Lucy early in the film in the car gets a call from "Lucy" that she doesn't answer. Her ringtone is an actual old phone ring, like from the past, or a movie. When she later pulls out her phone to show her paintings, she has a lot of missed calls from "Lucy" which then changed to "Luisa" in later missed calls. Of course, the paintings aren't hers, they're just paintings the Janitor, a failed painter, likes, her poem isn't hers but a poem he liked, which she discovers while investigating his bedroom. There's also a book of movie reviews by Pauline Kael in his bedroom, so when Lucy later goes off on an extended rant about A Woman Under the Influence, I figured that was Kael's review. The Janitor is eager to read and learn, but isn't confident in his abilities, and maybe not that bright, so lets the opinions of others that he reads to browbeat him, because they must be smarter than him, and thus must be right.

The bit with the Janitor in the truck is maybe him breaking down and getting hypothermia from sitting in the truck too long, which can ironically make one feel hot and shed clothing. The final song is his acceptance of his life.