When Shin Jaejeong's dad rents out part of their house, he loses his favorite room, and gains a grudge. The tenants? A single mom and her daughter, Woo Arin. Things spiral when Jaejeong discovers their parents are having an affair. And, turns out, Arin just learned about their affair too. Bound by this dirty secret, Jaejeong and Arin grow closer, even as resentment simmers. As secrets pile up and tensions rise at school, the two form a dangerous pact: if their parents won't stop the affair, they will.