Seohwa lands a job at the leading mattress company, only to discover that its CEO, Cheolsu Gam, is a sleep demon who's been awake for 600 years! When they first meet, he senses that she's his human partner – the sole individual capable of putting him to sleep and sending him back to the demon realm. Although they start off as colleagues, a strong attraction begins to grow between them. But will Seohwa's newfound inability to sleep prevent them from fulfilling their fate?