“Marriage is about tackling problems together.”

So says the hand-carved display in the small but cozy living room of Hyun-su (Lee Sun-kyun) and Soo-jin’s (Jung Yu-mi) apartment. What the struggling actor and his rising executive/very pregnant wife don’t know yet is that they’re about to have a hell of a problem to tackle.

Writer/director Jason Yu’s Sleep is a smartly scripted, playfully wearying horror with tension rooted firmly in how very much you like Hyun-su and Soo-jin.

At some point in Soo-jin’s ninth month, Hyun-su begins to talk in his sleep.

“Something’s inside.”

And then he walks in his sleep. Eats. Claws at his own face. This, obviously, becomes somewhat frightening, but the couple aims to tackle this thing together. Of course, soon enough there will be three of them.

Yu slowly cranks up tension as Soo-jin struggles between a maternal desire to protect her baby and a deep-rooted commitment to working through every marital problem with her husband.

One of the anxieties Yu toys with is that bone-deep exhaustion of a new parent, amplified for Soo-jin by her wakeful watch to make sure her husband doesn’t do harm to the baby in his sleep. You’re exhausted for her, and when she seems to start making rash, even insane decisions, well, who could blame her?

The way Yu manipulates tone is a thing of wonder. The more desperate and bleary eyed the film becomes, the funnier it is, and that dark humor is both at home and wildly startling. But there is a sweetness to it, and a camaraderie between Jung and Lee (who died tragically last year) that insists on your investment in the outcome of their story.

The third act is almost brazenly unhinged, and Sleep is all the better for it. It’s a tricky tale meticulously crafted, but it has a sweetness at its heart and that’s what makes it memorable.