A small film about a big subject, “Pu-239,” which airs on HBO Saturday, begins with a joke about life not being much better in post-perestroika Russia than it was under communism. It then introduces us to our hero, a nuclear engineer named Timofey Berezin, standing in a Moscow marketplace beneath a pristine portrait of Vladimir Lenin, complete with red hammer and sickle. We then briefly track back two days in time and start over again, with Timofey posed against the same poster, in case we failed to get the point the first time.

Who produced this movie? Oh yeah, Steven Soderbergh, George Clooney, and Peter Berg.

Timofey, who is played by the English actor Paddy Considine (who recalls a less rumpled Stephen Rea), isn’t feeling too good. Following an accident at a decrepit nuclear power plant so secret it cannot be found on either civilian or military maps, he’s ingested 1,000 RAMs of radioactive poison and knows he has at most a few days to live. The future of his wife, Marina (Radha Mitchell), and of his young son weighs heavily on his mind, particularly since he has been suspended from work without pay — his reward for having sacrificed himself to avert a larger disaster. The apparatchik who informs Timofey of this generous allotment looks suspiciously like Mikhail Gorbachev minus the glasses and the bald, ink-spot pate. Are the filmmakers trying to send us a subtle message? “Mr. Gorbachev, tear down that wall!” (“No, Mikhail! Wait! On second thought, put it back up!”)

Timofey, who deteriorates physically with every passing scene, has in some ways been a very fortunate man. True, he has spent 12 years working in a decaying nuclear plant, but he also has the beautiful Marina, with whom he has been in love since the day they met. Both are devoted to their son. Both are cultured and intelligent — scientists and readers. Both consider themselves, not without justification, to be moral people. Enter the dilemma. What if you, like Timofey, had only days to live (a fact you conceal from your wife), when, in a gesture of solidarity, one of the workers at your clapped-out facility secretly handed you a tiny package containing 100 grams of stolen, weapons-grade nuclear material, the implicit suggestion being to sell it pronto so that your wife and child will have enough money to survive after you’re gone?

That, anyway, is the setup. The larger political point, aside from the personal moral quandary, is that a variant of this kind of story is not beyond the realm of possibility. Timofey knows what kind of client such a package might interest: the North Koreans, someone in Afghanistan, or anyone extremely unpleasant with a desire to terrorize the world.

So at the Moscow marketplace (he has managed to convince his wife that he has gone there to look for a job), Timofey stands holding a large piece of cardboard on which he has written “Pu-239” in the hope that someone will know what it means. No one does, but Shiv (Oscar Isaac), a shakedown artist in the youthful Al Pacino mode — black hair slicked back, red leather jacket, thick silver bling around his neck — is curious enough to approach him.

Shiv, along with his sociopathic pals Vlad (Jason Flemyng) and Yegor (Jordan Long), are Moscow’s version of the Three Stooges via Quentin Tarantino and Martin Scorsese. They are employed by Tusk (Nikolaj Lie Kaas), one of the city’s premier racketeers who is often to be found, unshaven and in a black leather jacket, inside a nightclub in which nubile women do what they do in any nightclub worth its salt.

That Shiv, a moron with a sprinkling of decency in his heart, should have been the person to approach Timofey is highly unfortunate for our hero. But there’s no getting rid of him. Shiv and his pals have just blown up a store, which turned out to be the wrong store, and the terrifying Tusk is mightily ticked off about it. Either his thugs produce $6,000 for him within 72 hours or they’re dead. So even if Shiv has a hard time understanding exactly what Timofey is selling, he’s determined to convince Tusk that he’s got something far more valuable than $6,000 for him. (Timofey’s asking price is $30,000 — a mere fraction of the material’s worth.) And if Tusk won’t go for it, there’s always Starkov (Steven Berkoff), a tennis-loving hood with a fetish for cute ball girls and a strange obsession with the 140-mph serve of the robotic 1970s-era American tennis player, Roscoe Tanner.

But then everyone is besotted with America. Heading down the long escalator into one of Moscow’s famously deep subways, Shiv lectures his girlfriend’s son (who may or may not be his own son) on the great American outlaw Jesse James (hounded by the CIA, according to Shiv), then gets him mixed up with Jesse Jackson. Finally, confusing himself further, Shiv solemnly informs his young charge that the Jackson Five are the most powerful family in America.

Not all of this Tarantino-for-dunces banter suits the somber narrative of Timofey, whose own thoughts — for example, “Radiation is all around us. It is measured in RAMs. A person living in Denver receives .05 a year due to the altitude. A patient receiving a chest X-ray receives .01 RAMs” — are delivered in a funereal voiceover, and often involve metaphorical musings on isotopes, particles, waves, the periodic table, and other things you’re likely to know little about. Timofey is a pure man who has been fatally contaminated. All around him in Moscow are healthy, vicious thugs whose contamination is cultural, a Wild West of brutalist capitalism. What are the chances he’ll manage to sell his fissile material and send his wife and child the $30,000 before the radiation turns his innards into soup?

bbernhard@nysun.com