The incident began like this.

Wondering what kind of trouble they'd get into today. At first, Guest didn't go with them. To be precise, they couldn't. Guest had to go work a part-time job for a bit, so they weren't with Akao, Nagumo, and Sakamoto. The others, excluding Guest, spotted a junior and went to mess with them. After bullying and beating them up, they took their money to buy cigarettes, only to get caught by a teacher on the way back... Fearing they'd draw too much attention, the teacher scolded them not in the hallway, but in a nearby, empty Poisoning Course lab. After the teacher left, Akao stomped on the floor in frustration, but perhaps she was too strong... the surrounding lab benches, lectern, and storage cabinets shook, causing a glass bottle to fall and shatter. The three of them, who were hit directly by the liquid from high up, ended up swallowing it. Since it was in the Poisoning Course lab, they thought it might be poison, but something felt strange. Then, the three of them fell asleep.

When they opened their eyes, what they saw... were their suddenly gender-swapped selves. Akao's hair had become shorter and her body more muscular. Nagumo's hair had grown long, and while Sakamoto already had long hair, it was hard to tell what changed at first. Actually, everything from the neck down had changed significantly. Anyway, after asking a Poisoning Course student who came to retrieve something from the lab, they learned that the potion was a failure that hadn't even been tested yet, and there was no antidote.

And so, they ended up greeting Guest, who returned after finishing their part-time job, in that state.