Every week, we pick a new episode of the week. It could be good. It could be bad. It will always be interesting. You can read the archives here. The episode of the week for January 20 through 26 is “Twin Cities,” the sixth episode of the second season of Starz’s Counterpart.

A point of divergence is almost always some small thing you don’t even recognize — the right turn you took one day that let you meet your spouse, say, or the time you didn’t go down a path that would have led to your death (not that you would ever know it).

Sometimes you might narrowly slam on the brakes at a red light and see the car that otherwise would have hit you go whizzing through the intersection. But most of the time, we aren’t aware of the might-have-beens because they are too strange and complicated, too bound up in probability and unlikely scenarios.

And that’s to say nothing of the enormous web of decisions that stretches out on all sides of any of us, back into the past and forward into the future, all a series of choices made by other people to bring us to this particular moment, where we feel at once masters of our own destiny and trapped by the destinies of others.

What if you could see the might-have-beens, though? What if you were a scientist, and you could create an experiment where you built two realities, exact replicas of each other, and then started changing very, very, very small things in each of them, until that led to the rupture of tragedy? What if you were you but also somebody else? Those are the questions asked by “Twin Cities,” perhaps the best episode the Starz spy drama Counterpart has made yet.

“Twin Cities” is an elaborate flashback to how the entire world of Counterpart came to be

For those who aren’t familiar with the show, an explanation of Counterpart in brief: In 1987, East German scientists somehow created a gateway to another world, which was an exact replica of ours in every way. Same people. Same politics. Same circumstances.

Whether the world was a copy of our own or a universe that had always been running in parallel to ours without us being aware of it was unknown. What was known was that there was a whole other world over there, one that could be studied and experimented upon and maybe exploited.

Counterpart, barring a few flashbacks here and there, has always taken place in the present day of Berlin, following a man named Howard (J.K. Simmons) who meets his exact double on the other side. (The series’ nomenclature for the two universes is that ours is the “Alpha” universe, and the other one is the “Prime” universe. Therefore, Howard from the other universe is called “Howard Prime.”) From there, the series spins off into a dizzying series of butterfly effects, designed to underline the utter chaos of the strange Cold War that exists between the two universes.

“Twin Cities” is different. It features almost no one from the series’ regular cast, save for the mysterious Mira (Christiane Paul), who has been revealed throughout season two to be the daughter of the man who created the gate between the worlds in the first place, a scientist named Yanek (James Cromwell).

Yet present-day Mira and Yanek barely appear in the episode, which is largely set in the late ‘80s and early ‘90s and involves the initial opening of the gateway by Yanek (played by Samuel Roukin in the past), what he and Yanek Prime found there, and how the decisions they made thereafter created the icy paranoia of Counterpart’s present day.

The story that “Twin Cities” tells is one that someone who’s never seen a single episode of Counterpart could appreciate. Yes, it helps to know just why the series keeps flashing to present-day Mira and Yanek every 20 minutes or so, and if you’re a long-time viewer of the program, some of the revelations about the origins of the two separate universes will be more satisfying than if you’re not. (I should also note that for fans of the show who’ve been struggling with its ultra-dense plotting in season two, “Twin Cities” serves as a welcome respite from the “Now which version of which character was that?” questions that have littered the season’s first five episodes.)

But the episode is also a simple story about two men who are the same man, and about the same-but-different universes they came from. It digs into how the act of observing something irrevocably changes it, and how minor choices have unintended consequences.

The episode is Counterpart’s first to be directed by series creator Justin Marks (who also wrote the hour), and he nicely captures the eerie grandeur of encountering your exact double, in a scene where the two Yaneks first meet in a shadowy corridor, each dropping their flashlights, so the beams spin and point away from each other, a tiny moment of symmetrical chaos that portends everything to come.

Both Yaneks are family men, dedicated to their identical wives, identical sons, identical daughters. Both Yaneks were trying to defect to the United States, but ultimately stay in East Berlin because they can’t simply overlook this astonishing new discovery. But both Yaneks are not the same man, no matter how much they might postulate they are. And so they begin their experiment, which will open the gulf between them into a yawning chasm.

“Twin Cities” both buys into and negates the idea that we could build some better world by changing the past

The Yaneks’ idea for a carefully controlled experiment is simple. Yanek Alpha will buy a cassette tape for the music-loving Mira (played as a teenager by Emilie Neumeister). Yanek Prime will not. It will be easy enough for the two Yaneks to observe if anything important changes — but such a small change shouldn’t cause any major changes, right?

But Counterpart is a science fiction story, which means there are consequences. The East German secret police are after Yanek’s son, who has been spreading subversive flyers speaking against the Communist government. When they come to Yanek Alpha’s house to question the boy, one of them holds the kid down, ultimately choking and killing him, because Mira Alpha, busy listening to her new cassette tape, didn’t step out of her room in time to see that her brother was near death. Mira Prime, without her cassette, heard the police immediately and intervened, so her brother lived. (Marks stages these two outcomes in a splitscreen that underscores the sheer random chaos of the event.)

Yanek Alpha, then, descends into grief, slowly but surely losing his grip on his mind and his family, while Yanek Prime has an undisturbed life where he gets to see both of his children grow up. Soon, Yanek Alpha is crossing over to the Prime Universe to spend time with his family, to pose as his doppelganger, to try to recapture what he’s lost. Wouldn’t you?

Along the way, “Twin Cities” contains plenty of Easter eggs for Counterpart superfans, like a reveal for why the two sides communicate via a series of elaborate messages passed via outmoded technology and a mention of a very significant plague that will devastate the population of the Prime universe. But these elements will play just as well for those who’ve never seen the show before, because they come up organically in the story of the two Yaneks trying to control this strange and terrible anomaly and, instead, being consumed by it.

Marks also plants a kind of shadow series within the series he already has with several scenes where Yanek and some of his scientist colleagues (from both worlds) work together to understand the gateway. The scenes, filmed with a combination of body doubles and actual sets of twins, have a fun, nervy feel to them, like they could have been a sci-fi spinoff of the show proper.

But we know that sci-fi spinoff can’t exist. The present-day world of Counterpart is so different from this past one, because that little choice to give one Mira and not the other a tape, so seemingly insignificant, created two entirely different worlds. Grief follows, as do murder and suspicion. And the implication is that if it hadn’t been the cassette tape, it would have been something else. The universe is always trying to change.

By the end of “Twin Cities,” Yanek Alpha, slowly descending into a haze of paranoia, finally kills Yanek Prime, only for Mira Prime (who as yet doesn’t know the two worlds exist) to see one version of her father drenched in her actual father’s blood. (This is one way to learn you live in a science fiction universe.) It sets her on the path we know she will remain on in the series’ present day, when she longs to destroy the Alpha universe for good and save her own.

But this is the sinister lie at the center of Counterpart. The world is broken, it says. Both worlds are broken. If you could change one tiny thing, you might put everything back together again. But there is no easy way to heal anything. All choices lead, eventually, to pain.

Counterpart airs Sundays at 10 pm Eastern on Starz. Previous episodes are available on Starz’s digital streaming platforms.