VENICE, Italy—Have you ever discovered the person you trust most in the world is being unfaithful? There are infinite ways of dealing with this discovery. The most obvious would be confronting the person directly, but that’s a tough thing to do—and it can end terribly. People seek revenge, get violent, and desperate. But what if there was someone who could do the work for you, a person you can pay to come in and end the affair before it ruins your life?

Elizabeth Lo’s fascinating documentary Mistress Dispeller, which just premiered at the Venice Film Festival, explores that very idea. In China, if someone has a suspicion their partner is cheating on them, they can hire a mistress dispeller. Their job is a tricky one: They integrate themselves within a couple and bring the affair to an end.

Mistress Dispeller follows an older woman who’s been married to her husband for decades. On the surface they have it all: a big apartment with a great view, a grown, successful son, and they’re financially secure. But their relationship has taken a turn in recent months, and when the woman discovers a text on her husband's phone, she discovers he’s been........

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