When it comes to a tough sell, infanticide is hard to top. As a dramatic concept, the topic is destined for “Oprah,” or else handy as a time-honored theme for a country ballad. Thus the director Hilary Brougher wins big points for having the guts to make “Stephanie Daley.”

Ms. Brougher gets bigger points for avoiding the obvious and using the template as a prism through which to explore human nature. The film is less a movie-of-the-week social drama than a philosophical investigation into the universal mysteries of motherhood, faith, fidelity, and biological caprice.

Tilda Swinton, an actress of almost preternatural resources, plays Lydie, a forensic psychologist assigned to the case of the so-called “ski mom”: a 16-year-old girl named Stephanie Daley (Amber Tamblyn) who has been charged with murdering her newborn daughter after delivering her in a public bathroom during a high school ski trip. In a narrative flourish that quickly establishes the film’s parallel structure, Lydie is six months pregnant, anxiously awaiting her baby’s arrival in the wake of a previous pregnancy that ended with a stillbirth. The psychological shadings are complex and often troubling but also drawn with an extremely subtle touch. What might come across as contrived instead becomes engrossing, as the camera methodically tracks as much interior space — the characters’ minds and hearts — as detailed exposition.

Lydie’s job is to get Stephanie to tell her what happened: How did she wind up having a baby when, as she claims, she never knew she was pregnant? And once she held the infant in her hands, was it stillborn or did she kill it? Although the prosecutor has ample evidence to convict the child, Lydie must decide whether she’s competent for trial and if she really believes the truth of what she’s confessed.

There’s an element of “Law & Order” procedural here, but mostly the film juxtaposes the women’s stories. Through flashbacks, Ms. Brougher shows how the shy Stephanie is seduced — it’s more like date rape — by a college boy at a keg party, before deciding that she couldn’t have gotten pregnant because he claims he didn’t ejaculate. As Stephanie proceeds through the course of the pregnancy, her breasts grow bigger and her periods stop, but she continues to rationalize away her situation, even as she sinks deeper into a desperate fear.

Ms. Tamblyn, who previously played a TV teen who heeded direct messages from God (“Joan of Arcadia”), here portrays a girl for whom religious instruction seems ill-applied, perhaps even encouraging her behavior. At one point, in an odd bid to encourage abstinence and prep kids for parenthood, Stephanie is given a beeper egg in health class — the idea is for students to learn nurturing skills. When it goes off in the car, she explains to her mother, “when it beeps, you press the button so you don’t get pregnant.”

As Lydie digs deeper into Stephanie’s case, she is gradually overtaken by her own sense of disquiet. Her husband (Timothy Hutton) is working late at the office and arriving in bed with Scotch on his breath, and there’s the matter of a single earring discovered in the cat box. Lydie, it is noted early on, has never pierced her ears. But there’s something else that we don’t know about, apparently, which binds the shrink to her subject in a way that is both symbolic and symbiotic.

Ms. Swinton, whose lanky elegance is matched by the seismographic sensitivity of her face, inhabits her character’s thoughts so richly that they can be read without dialogue or much action. The camera often sits still for entire scenes, or hovers in for extended close-ups in low light — often cutting away to something seemingly incidental to the scene. Ms. Brougher is highly conscious of connecting her protagonists to the natural world. Her eye lingers, in poetic fashion, on little things that loom with portent. There are fleeting moments when the film tips into the moral phenomenology of a Kieslowski episode.

The director also knows how to present material that is emotionally grueling without flinching or wallowing. The climactic flashback is jarring, yet rendered with skillful narrative choices that take us inside Stephanie’s psyche rather than merely illustrate her predicament. It’s a great performance, and a film whose ultimate spirit of compassion will stay with audiences for some time.