The protagonist of the novel, Hitomi Hirosuke, is a trainee teacher who commutes to a certain girls' school, as he hopes to earn his stripes and graduate to the rank of staff teacher. Every night he sees almost the same kind of nightmare where an angel with the face of a girl is being violated and devoured by a tentacle monster. Every day he visits the school's infirmary where he discusses his nightmares with the school nurse, Oomori Tonae, while smoking. Meanwhile, his stress isn't helped by the fact that his strict and no-nonsense supervisor and mentor, Tashima Semina, requires him to show up for a daily review in her office with a performance report in hand. One day, however, he bumps into one of the students, Sugamo Mutsuki, whose face he seems to recognize from his nightmares...
As the nightmares continues to haunt him every night with increasing intensity, Hirosuke discovers to his horror that his grasp on reality is slowly, but surely starting to buckle under the pressure, as the horrible imagery from his dreams begins to follow him into the waking world as disturbing visions and hallucinations. As Hirosuke tries to carry on with his job, despite his crumbling mental health, he finds it harder and harder to tell the difference between reality and delusion.
This work makes use of the following tropes:
- Childhood Friends: Tamachi Mahiru, a girl Hitomi runs into at the school patio, claims to be his closest friend when they were kids.
- Cuckoo Nest: At the end of the story, Hirosuke is revealed to actually be a patient at a mental hospital, where he is undergoing treatment for a severe depressive psychosis. It turns out that the school is said hospital, that Oomori Tonae is not a school nurse, but the doctor in charge of his treatment, and most of the various other characters he has been interacting with throughout the story have been objects and animals he has found around the facility and anthropomorphised as a part of his delusions. Finally, Sugamo Mutsuki is another patient at the facility, whom he has taken a shine to.
- Fan Disservice: The majority of the hentai scenes in the novel are actually this, and probably only really qualify as "sex scenes" in the strictest technical definition of the term. They are squarely and uniformly Played for Horror, as they are Hirosuke's violent and increasingly depraved fantasies of inflicting harm on the women around him, rooted in a combination of his mounting stress, rapidly decaying sanity, and general sexual frustration.
- Foreshadowing: The girls never once stray from their designated spots. They can't, because four of them aren't real people: Miyuki is a preserved brain sitting on the library windowsill, Koyori is an archery dummy, Mahiru is a cat, and Nozomi is an injured crow. Mutsuki is the only actual human; the empty classroom is probably just her hospital room.
- Gorn: The bad endings, in abundance.
- Gratuitous French: In the title.
- He-Man Woman Hater: As Hirosuke undergoes his Sanity Slippage, his thoughts gradually become more and more overtly misogynistic, with many of his fantasies and delusions centering around wanting to assault and inflict gruesome harm upon the women around him.
- Mind Screw: That's characteristic of the denpa subgenre.
- Morally Ambiguous Doctorate Thanks to the Cuckoo Nest reveal at the end, this is at the very least Implied about Oomori Tonae. It is hard to not read details from her interactions with Hirosuke throughout the story that implies that she, as the doctor in charge of his treatment, has not only been enabling and feeding into his delusions — which is something a supposed mental health professional should never do — she might even have had sexual encounters with him at several points, something which clearly counts as sexual assault, as Hirosuke is both her patient and the nature of his mental illness means that he is in no position to give any kind of informed consent.
- Pretty Boy: Though the Player Character, Hitomi Hirosuke, is never seen clearly on-screen during the story, with his face always being out of frame whenever he makes an on-screen appearance, the game's character designer and main artist, Kenzo Nagaoka, has since shared concept art of him online, showing that he is a troubled- and exhausted-looking, yet still quite ruggedly handsome, young man somwhere in his late-twenties, with a Looks Like Cesare kind of appearance to him.
- Sanity Slippage: As with most denpa stories, the plot sees the Player Character Hitomi Hirosuke slowly losing his sanity and becoming unable to tell reality and his increasingly intense delusions apart.
- School Nurse: Oomori Tonae, the usual smoking partner of the protagonist as well as the only person he even tells about his grotesque nightmares. It turns out that she is actually one of the doctors working at the psychiatric facility he has been committed to, and the doctor in charge of his case specifically.
- Tentacled Terror: This combined with Naughty Tentacles is what Hitomi sees in his restless dreams every night.
- Trope Codifier: The whole denpa subgenre phenomenon had not yet entirely solidified as a proper genre of its own by the time Sayonara o Oshiete was originally released, and as such it wasn't intentionally written as a denpa work or taking any direct or overt inspiration from its contemporaries. On the whole, the denpa genre's roots can be traced all the way back to the late 1980s, and there are also quite a few other visual novels playing with many of the same themes as Sayonara that just slightly predates it. That said, Sayonara's use of the Sanity Slippage trope in regards to its protagonist, Hitomi, and how it was communicated to the player, resulted in it ending up becoming a central work that played a large role in helping sharping and popularizing the genre.
- Unwitting Instigator of Doom: On the third day, Mahiru tells Hirosuke about the "Seven Mysteries of the School", including a certain tree where an angel is said to lay dormant. Unfortunately, because Hirosuke's recurring nightmares are of an angel being assaulted by a monster, learning this causes his delusions to begin spiraling out of control.
