A marriage is tested after expectant mother Soo-jin (Jung Yu-mi) awakens one night to the eerie sight of her husband Hyun-su (Lee Sun-kyun) sitting on the end of the bed, forebodingly muttering “someone is inside” before collapsing into a steady snore.

In the days that follow, Hyun-su’s nocturnal behaviour further devolves into increasingly disturbing spells of sleepwalking, and the couple’s once-idyllic relationship begins to deteriorate under the mounting paranoia that Hyun-su might inadvertently hurt himself, Soo-jin, or his unborn child. Their family doctor diagnoses the condition as an extreme case of REM sleep disorder and prescribes medication. However, an encounter with an eccentric shaman sows the seed in Soo-jin’s now fraying psyche that her husband’s transformative symptoms may have a paranormal source.

Having worked under South Korean luminaries like Lee Chang-dong and Bong Joon-ho, writer-director Jason Yu appears to have inherited their propensity for shaping character, subverting genre, and imbuing existential dread. Further demonstrating incisive storytelling instincts, nary a frame is wasted across the film’s tight three-act structure, which devilishly ratchets up the stakes and shifts perspectives with each uncanny episode. And so, with nimble finesse, Yu’s deft direction sets the stage for a disarmingly frenzied confrontation that balances humour, suspense, and a potent ambiguity, all brilliantly performed by his stellar co-leads and an adorable Pomeranian.