Steven Soderbergh, rightly considered one of Hollywood’s smartest movie makers, is at his cleverest in “Side Effects,” a canny, cunning big-idea thriller in a minor key, an engrossing zeitgeist whodunit about Wall Street, Big Pharma, prescription drugs and the power we give psychiatry and psychologists.

Put simply, it’s about a death, perhaps caused by a depression drug.

Channing Tatum plays a Wall Street type just getting out of prison for securities fraud. Rooney Mara is Emily, his seemingly overwhelmed wife, a morose beauty burdened by the responsibilities she now carries (she’s the one working) and the memory of the life they lost.

Her attempted suicide-by-car puts her in the care of Dr. Jonathan Banks (Jude Law), a seemingly sincere psychotherapist who must come up with a drug — on the fly — that keeps a distraught but apparently sane Emily out of the hospital.

How about this new thing, the one she’s seen the ads for on TV — Ablixa?

“Side Effects” nicely describes the slow-moving “fog” of depression as waves of it overwhelm Emily. And it plays like a case study in her treatment by drugs — her downcast behavior replaced by other manifestations of “not quite right.” Suddenly, she has energy and can function at work and in her marriage. But she’s up at all hours, sleepwalking. She forgets things.

Then, a crime happens and we wonder whether the cause was the drug, the doctor (legally paid by the pharmaceutical company to test the drug), the patient’s predisposition (Catherine Zeta-Jones plays her previous doctor) or something else.

Soderbergh elegantly suggests that if you’re suicidal, every trip down to the subway, every boat ride in the harbor, every moment behind the wheel or meal prepared with sharp knives has the threat of an impulsive, irreversible act.

Soderbergh, working from a Scott Z. Burns (“Contagion”) screenplay, transforms this tricky mystery into hints of something trickier — Wall Street machinations, the murky relationship between therapists and the courts, between doctors and pharmaceutical companies.

“Side Effects” loses momentum near the end, piling on implausibilities. But even then Soderbergh never lets things slip from the implausible to the impossible.