Even though I could date the school's most beautiful girl, Plain Girl is just too erotic

With the long-awaited girlfriend finally in sight, the protagonist's school life was promised to be smooth sailing. - However, at that time, the protagonist, due to a certain motive, failed to notice the spider creeping up on him... - But that's understandable— - Because that spider was a plain-looking girl who would normally go unnoticed... - Yes, just an ordinary, unremarkable woman...except for one thing...​