Nam Hae-sul, who is handsome but famous for his dog-like personality, is only nice to his childhood friend, Yang Mi-ru. "It's clear that he's nice to me because he likes me!" But is it really like that? Miru has no idea about Haesul’s true feelings. Then one day, she finds out about the 'black heart' that Haesul harbors! Cracks are starting to appear in their 10-year friendship...! The wild growth story of a wolf boy and an innocent girl!