Themes of guilt and atonement hang over this Belgian drama, written and directed by sibling filmmakers and starring Adèle Haenel as a doctor who starts acting as a detective. (Sundance Selects)

A mystery lies at the heart of “The Unknown Girl,” but it isn’t the one you think. When a young sex worker (Ange-Déborah Goulehi) is found dead under suspicious circumstances near the office of Jenny Davin (Adèle Haenel), a young doctor working in Belgium, Jenny is plagued by the thought that she might have prevented the death — if only she had opened the locked door to her practice when the unidentified woman rang her bell, apparently seeking assistance. But it was late at night, the office was closed and what has been done cannot be undone.

Or can it?

The themes of guilt and atonement hang heavy over this quietly mesmerizing drama, written and directed by sibling Belgian filmmakers Jean-Pierre and Luc Dardenne (“Two Days, One Night”). Jenny quickly becomes obsessed, not with solving the crime — if one has even been committed — but with finding out who the dead woman was.

Make that who she is. As Jenny later tells someone — a character who turns out to be similarly haunted by the event, and for reasons that are not entirely different from Jenny's — “If she was dead, she wouldn’t be in our heads.”

As Jenny takes the investigation into her own hands, she becomes part sleuth and part therapist, a hybrid performance that’s rendered by Haenel with a kind of ferocious yet tightly controlled focus. That same quality of professionalism tempered by compassion makes Jenny, as a doctor, beloved and a bit of a cool customer. As a detective, she’s more dogged and diligent than the actual cops.

But the role that suits Jenny best is as a confessor. In the end, it isn’t her own shame at her inaction that fuels this low-key thriller, but her ability to elicit the unbosoming of others. The question that looms large here, lingering long after the closing credits, is whether, despite our human need for forgiveness, absolution is ever truly possible.

Unrated. At the Angelika Pop-Up at Union Market. Contains strong language and mature themes. In French with subtitles. 106 minutes.