Hae-joon (Park Hae-il) and his wife live apart on weekdays, with him working in Busan, and she in the seaside town of Ipo. When Hae-joon is sent to investigate the death of a man who fell from a mountain top, he finds himself drawn to Seo-rae (Tang Wei), the man's Chinese widow who is beautiful, mysterious and doesn't seem particularly sad about her older husband's passing.
The chemistry between the two is undeniable, with Hae-joon – a serial insomniac – now even able to get some sleep. But Hae-Joon's judgement begins to cloud. Is Seo-rae playing him or not?