The movie is about an elderly man who presumably has never had a girlfriend and dropped out of society (beyond having a janitorial job) in his 20s.
The narrative in the movie itself takes place within the elderly man's imagination, as he pays one last visit to his memories before he dies. Alongside him is a succubus, who is both his anima (in Jungian terms, every person contains an Anima and Animus, male and female component), and also an idealised girlfriend he never had (he describes her early on as "ideal", jokingly).
They drive through heavy snow and she sees a brand new swingset in the garden of an abandoned home, and this suggests the following scenes are going to be a similar mix-up of memories, their chronology confused. Jake takes his girlfriend to the family barn, where he mentions that his father found these pigs which had been rotting for a long time without anyone knowing, eaten alive by maggots. This is both a significant memory for Jake, but also symbolic of his own later life, where he goes about his daily life but is rotting inside, eaten alive by the maggots of regret and despair etc.
The family scenes in the farm house shows us that Jake is somewhat autistic / avoidant and that he is nonetheless caring towards his elderly parents as their health deteriorates. It is also suggested IMO that Jake is a momma's boy, both because his mother is there on stage at the end of the movie clapping and also because in the farm house his father says to the girlfriend that she may want to put on some of Jake's mother's lingerie so she and Jake can fuck - this suggests an oedipal complex Jake wasn't able to transcend or overcome, essentially remaining a semi-childlike guy throughout his life.
The basement I am not quite sure about, but it is tempting to think of it as Jake's subconscious, a part of his psyche which he may have repeatedly tried to enter in the past to find out what was wrong with him (hence the scratches) or may have since taped up and locked (hence the selotape on the door) because it is too painful to confront the truths of his life. In the basement is the revelation of who he is, beyond the fantasy that the film represents. Here is the fact that the paintings his "girlfriend" claimed to have painted are in fact his own, a product of his own repressed side, or his anima, an ambition he never had the courage or willpower to pursue, a memory of a time he was young enough to do so now repressed. Here is the brutal fact of his life, which he has tried his best to distract himself from, namely that he is an elderly janitor living alone (e.g., the succubus pulling the janitor's uniforms out of the washing machine).
We then leave this house, Jake having paid one last visit to his parents, recalled them at various stages of their lives, reminded himself that he wasn't all bad considering he cared for his mother and father late in life, recalling the family dog, his old childhood photos, his bedroom, etc. They drive to a dairy queen-esque place in the middle of the night, in the middle of a snowstorm, and this I believe represents a few things. The first is that this is a place Jake always thought about bringing a date, but never did; maybe it was the local place where sweethearts came to hang out (later on, Jake finds a garbage can full to the brim with empty cups from that dairy queen, suggesting he has revisited this romantic date fantasy many times). Second it is an opportunity to see how Jake communicated with people outside his family, including the popular succubi (whose faces he can't remember from childhood, so he simply uses the faces of succubi at the highschool where he is a janitor), and also a shy succubus who may have showed some sympathy to him. Again, due to the nature of the movie, and the blurred boundaries between Jake and those he imagines being close to, I believe that the rather shy succubus who serves his girlfriend is both a memory of Jake himself in highschool (hence the matching skin rashses) and also maybe a succubus he himself overlooked or was too shy to talk to, someone nice. This succubus says to the girlfriend that they don't have to "go forward", meaning that unlike other romantic fantasies where Jake has stopped at this Dairy Queen, this succubus knows there is something amiss, this succubus being a small compassionate aspect of Jake's personality which does not want him to kill himself, but she nevertheless wishes them luck as Jake makes the final journey to his death.
The highschool serves both as the literal workplace where Jake has served for presumably decades, as a reservoir of memories (in the sense that when one looks back to one's youth, the school inevitably looms large), and also a place where Jake both literally and psychologically returns to on a regular basis. When Bruce Springsteen began driving to his childhood every night while depressed, a psychologist told him that it was his unconscious mind yearning and perhaps naively believing that he could go back in time and change what happened here. The same applies here as Jake pulls up outside the symbol of his youth, a point in time when life was ahead of him instead of behind him, a journey he has obviously made in his imagination hundreds of times considering the aforementioned garbage can full of ice cream cups. Here in the car (a nice shot from above here from Kaufman IMO references the movie poster for Eternal Sunshine of the Spotless Mind, but rather than two lovers being beside one another near a crack in the ice suggesting a cracked or damaged psyche, the car itself occupies the position of the crack in the ice - i.e., the damaged psyche - while the place where Joel and Clementine lay is now symbolically bare) we see Jake and the succubus kiss one another for the first (and last time). This is Jake's last imagined kiss, and also the symbolic union of his anima and animus, the union also of those two parts of his psyche which are contesting the notion whether or not he should kill himself. An agreement has been made, a decision made, and there is no going back now for Jake, no more debating whether or not he is going to kill himself. When the succubus leaves the car and finds it locked upon trying to re-enter, it is symbolic of this final decision. She can now only enjoy the final few moments before suicide.
Inside the school she hides from the janitor who is mopping up, the janitor being both the literal embodiment of Jake in real life at his job after school, but also Jake mopping up his memories, cleaning his tortured mind before finally putting himself down. The janitor discovers the succubus, who is no longer scared and who for a few moments becomes the embodiment of a specific succubus who Jake has thought about all this time, a succubus he met briefly at a quiz night but didn't have the courage to ask out. Jake imagines what the succubus may think about him at this stage of her life so many decades later, and the girlfriend voices these thoughts and he accepts that realistically she probably hasn't thought about him once since then. There is a final dream scene, now detached from Jake's own experience, suggesting it is a kind of fevered, disembodied dream, where he dances through life with an idealised succubus, proposes to her, etc, and this dream is then interrupted by a sinister janitorial figure who fights to the death with the imagined version of Jake. These are the very last moments of Jake's psyche as his real-life character almost freezes to death in his truck and then ambles into the school one final time to stab himself to death.
I really enjoyed this movie.