Seohwa finally gets hired at the best mattress company in the country, but its CEO, Cheolsu Gam, turns out to be a sleep demon who hasn't been able to sleep for 600 years! Upon meeting, he suspects that she's his human partner - the only person who can make him fall asleep and allow him to return to the demon world. It's strictly business, but there’s undeniable chemistry between the two of them. Will he be able to go back to the demon world, or will her sudden inability to sleep thwart their destiny?