Contains spoilers for this episode and earlier episodes.
Ok-ran's birthday visit brings her inside Moon-young's house, where she claims to admire her mother's writing and handles a sharp object. Moon-young cuts her hand trying to take it away. Gang-tae arrives after Ok-ran has gone and embraces Moon-young in relief. Their argument over his rejection ends with a kiss and a birthday wish. When his fever worsens, she looks after him, and he admits how unfamiliar that care feels. Gang-tae explains that Sang-tae witnessed their mother's murder and has associated butterflies with danger ever since. He wants to remain beside his brother while also making room for Moon-young, rather than choosing one relationship over the other.
Moon-young asks Sang-tae to return and help illustrate a story inspired by his drawing of their journey. He worries that she will take Gang-tae away, while Director Oh's insistence on butterflies in the unfinished mural raises another conflict. The brothers fight after Gang-tae presses Sang-tae to face his fear and express himself. Later, Sang-tae hears Gang-tae say in his sleep that he likes Moon-young and recognizes a happiness he has rarely seen on his brother's face. He invites Gang-tae to dinner, pays the bill and gives him an allowance as an older brother. When Moon-young asks for a big brother too, Sang-tae calls both of them to follow him home.