In most alternate-universe fiction, the world is different from ours, and the audience knows why. AU stories are thought experiments, where a writer changes one piece of the world, then follows through the logical changes that result. What if the Nazis won World War II? What if the Confederacy won the Civil War? More frivolously, what if the crew of the Starship Enterprise were all bad guys? The fun of AU stories comes from the reasonable working-through of the scenario — the feeling that if one domino tips over, the rest will fall with an inevitable grace, remaking the world in a radical new form.

Initially, the new STARZ series Counterpart looks like a typical AU narrative. But as the series unfolds, it quietly undermines the basic premises of AU fiction. The alternate universe in Counterpart diverges, but it doesn’t diverge logically. There is no one explanation for why things are different between here and there. Rather than making the world more explicable, in Counterpart, AU makes the world more mysterious. That first domino got knocked over, and rather than falling in sequence, all the rest evaporated into mist.

Counterpart takes its time playing with AU expectations. The first episode opens slowly, following the life of Howard Silk, played with gleefully avuncular brilliance by J.K. Simmons. Silk is a minor bureaucratic functionary in a mysterious office in Berlin. His wife, Emily (Olivia Williams) was recently injured in a car crash. He’s a sweet man whose career is going nowhere. After 30 years, he’s just been turned down for a promotion.

But then another Howard turns up. This one is tough, mean, successful — and instantly distinguishable from the other Howard, because Simmons plays him with such swaggering, barely restrained bile.

Mean Howard also comes from another world. Nice Howard isn’t aware he’s been working for an organization that monitors a passageway between universes. Sometime around the end of the Cold War, our world discovered their world, or their world discovered ours, or the two worlds split apart. The exact mechanics are unclear. But the result is that there are two worlds which were the same until 30 years ago, and now aren’t.

Commerce between the two sides is secret and strictly regulated, which provides the background for a delightfully twisty spy plot involving assassins, secret cults, identity swaps, and embedded moles. It’s a science fiction variation on Russia / US tensions, with parallel worlds standing in for Cold War adversaries, with first rate acting and writing.

The unique part of the series, though, is the way the labyrinthine espionage, with all its uncertainties and obscurities, is layered over deeper, half-seen questions about Howard’s identity, and about the workings of his world.

Nice Howard and Mean Howard share the same DNA and the same childhood memories, but they’re radically different people. Some of the discrepancies are obvious. Mean Howard is a successful asshole spy, brimming with confidence and competence, while nice Howard is an amiable nonentity and a good person. But the differences go beyond that. Nice Howard likes music and good food; Mean Howard doesn’t listen to music, and needs to watch his cholesterol. Mean Howard wears black; nice Howard wears brown. They still like the same ties… though Mean Howard lost his favorite one years back. (He borrows Nice Howard’s with some glee.)

In a normal AU story, these differences might be about different choices. There’s some indication that a different decision by Emily Silk led to her husband’s different fates in the two worlds, for example. Maybe Howard and Howard are different because they, or someone near them, decided to be different. If that’s the case, then, as in the typical AU story, the world is at least somewhat logical or rational. Taking the road less traveled means you end up as a superspy; take the wrong one, and you become a cog in an indifferent machine.

Admirably, though, Counterpart never settles for that easy AU trope. Instead, many of the differences between the worlds seem actuated not by choice, but by pure chance. A main character in one universe miscarried; in the other universe, she didn’t. Why? In the six episodes provided for preview purposes, there’s no answer, any more than there’s ever an answer for why illness strikes one person and not another.

Similarly, an assassin in one world (Nazanin Boniadi) is a concert violinist in the other. The stark break suggests some sort of trauma — and in fact, the assassin did see her father die in front of her when she was a small girl. But the violinist saw the same thing; their paths only diverged afterward. Viewers don’t know why one is a killer and one isn’t.

Even the two worlds have radically, unaccountably different histories. In the counterpart universe, a worldwide flu epidemic wiped out millions of people. Many in the counterpart world believe there’s an explanation for the flu epidemic. They blame our world for spreading it. But that’s a conspiracy theory; there’s no evidence for it. People want to explain their worlds, but that doesn’t mean they can.

At one point, Mean Howard asks Nice Howard why he never advanced further in the company. His accusatory tone implies that Nice Howard lacked gumption or drive. Mean Howard wants to think he succeeded by brains or skill or grit. But Mean Howard’s brains, skill, and grit are the same as Nice Howard’s. Maybe there’s no reason one Howard succeeded and the other failed. Whether we get that promotion or fail to get it, even whether we’re good or bad people — a lot of seemingly major choices may just be random. Maybe we don’t choose this road or that road, but stagger down them blindly.

It’s possible that Counterpart will pull away from those insights as it nears its conclusion. As most AU fictions demonstrate, humans have a powerful urge to connect cause and effect. By the end of the series, we may know exactly the moment at which the Howards diverged, and who is responsible for releasing that flu virus. Perhaps everything will be explained. By midway through its run, though, Counterpart has done a remarkable job of refusing to provide its viewers with typical AU answers. There are no simple answers that unlock the universe.

Most AU is intended to illuminate the human condition, and the frailty of fate; it shows what the world could be, in order to throw into relief what is. But as Howard stares at his counterpart, everything dissolves into a fog of uncertainty. Finding a new world leaves everything less clear, not more. When you look into an alternate universe, Counterpart suggests, you find out how little you know about this one.