Honestly, trying to pin down a single 'main plot' for 'Secret Class' is a bit like trying to explain the plot of a fireworks show—it's dazzling, messy, and fundamentally about the spectacle. The core setup is this guy, Dain, who becomes a live-in tutor for a family after his parents pass away. He's supposed to teach the daughters about... well, life and relationships, because their dad thinks they're too sheltered. But the 'class' quickly becomes a front for a harem-style series of increasingly explicit encounters. The plot is essentially a vehicle for adult situations, with very thin narrative threads about Dain's past or his mysterious connection to the family sometimes bubbling up. I keep reading mostly out of a weird curiosity about how far the art style will push certain scenarios, not for any gripping storyline.
It's less about a traditional plot and more about a progression of fantasies. Each story arc usually focuses on a different female character—the mature aunt, the tsundere sister, the innocent younger one—and Dain 'educating' them. Any attempt at an overarching mystery or emotional depth feels like an afterthought. If you're looking for a manhwa with a complex, driving narrative, this ain't it. But if you want a straightforward, visually focused adult comic with a consistent premise, it delivers exactly that.