She persuades him to look after her cat while she is away on a trip and the two end up hooking up when they meet to sort this out. With Hae-mi gone away Jong-su waits eagerly for her return, diligently feeding the cat, which he never sees.
When Hae-mi flies back into the country she introduces Jong-su to urbane affluent trustafarian "Ben" who she met on vacation and it's very quickly apparent that he's been replaced romantically.
Regardless, Jong-su awkwardly continues to hang around with the pair, getting a taste of a life he can never have until one day he comes to suspect that Hae-mi is not safe with "Ben".
Long, this tight atmospheric thriller uses its length to frame Jong-su's life despite him being quite passive and having relatively little dialogue.
You see him watching the words and deeds of those around him and his actions make sense without the need for exposition on his part. Nominally unrelated things like a court case involving his father and the arising need for him to work the family farm are an important part of the whole.
This was Korea's (unsuccessful) submission to the Academy Awards the year before Parasite made history. It's a much less showy affair with less shock value but it is better than many an Oscar winner. Based on a Murakami story story this is a proper brooding thriller worth a feeling of mounting doom, highly recommended 9/10.