The crux of Counterpart, Starz’s surprisingly gripping new drama, is that 30 years ago, East German scientists in Berlin accidentally cloned reality, creating a parallel dimension accessible only via a dank cellar underneath a shadowy UN agency. It is, in other words, completely loopy. What makes the show work is that it invests very little in selling the science of this concept, choosing to craft a taut, suspenseful spy thriller rather than a more fantastical work of science fiction. The point of the show isn’t the portal itself; it’s the diplomatic intrigue that’s sparked by a world splintering into two very different realities.

What also makes Counterpart work is the Oscar-winner J.K. Simmons, who brings gravitas and emotional heft to a project that could easily careen off the rails toward crazytown. Simmons plays Howard Silk, a mild-mannered civil servant late in his career who’s completely oblivious, like most citizens of Earth, to the mind-bending secret in the basement of the organization where he’s spent 30 years as a pencil pusher. Howard clocks in early every day and visits his comatose wife, Emily (Olivia Williams), at night. But his innocence is shattered when he’s confronted by Alternative-Howard, a defector from the other dimension who will only work with … himself.

It’s a smorgasbord of different concepts in one. The Berlin setting, the espionage plotlines, and the deceptively ineffectual protagonist bring to mind the novels of John le Carré, but there are also elements of Homeland, Harry Potter, and Stieg Larsson thrown into the mix. Information, as any intelligence agency knows, is currency, and Counterpart is precise in what it reveals, eking out twists in each episode, but keeping the larger narrative under wraps. Howard and Alt-Howard are revealed in the first episode to be shockingly different: One is a gentle but  unremarkable soul while the other is a hulking, James Bond–style enforcer. Simmons embodies them perfectly, playing two totally distinct characters who somehow look identical. It’s a bravura performance that throws yet another question into the mix: What on either Earth happened to these two Howards to make them so different?

It’s notable that Counterpart borrows so heavily from existing tropes and yet feels so fresh. The first episode, directed by Morten Tyldum (The Imitation Game, Passengers), establishes that this is ambitious, expensive television—Howard’s workplace has the pallid, monochrome vibe of an adults-only Ministry of Magic, and the aesthetic of Alt-Berlin, an uber-modern city with hints of dystopia, is enthralling. It’s obvious early on that the two Howards are going to upend each other’s realities while also influencing one another. In the meantime, both are charged with investigating a larger conspiracy between the two worlds, and finding a stowaway in original-Howard’s reality—an assassin (Sara Serraiocco) who’s murdering agency employees one by one.

Serraiocco’s character has palpable shades of the fictional hacker Lisbeth Salander, and the Italian actress brings surprising vulnerability to the role. The supporting cast also includes Homeland’s Nazanin Boniadi as a mysterious wrangler, Ulrich Thomsen (Banshee) as the grizzled director of operations at Howard’s agency, and Richard Schiff (The West Wing) as a diplomat who represents his world in negotiations with its double. These are the moments when the show is at its most riveting, when the infinite questions its premise presents are answered. How do the two worlds differ? How do they get along? Why does one look like Berlin and the other like 22nd-century Shanghai? Why are smoking and hand-shaking illegal?

There are enough loose threads in Counterpart to make a tapestry, but the series seems committed to tying them together. Plus, for a show whose episodes run around 55 minutes, the pace is gratifyingly propulsive. It’s Simmons, though, who steals it, doing double duty as a milquetoast accountant and a braggadocious intelligence operative who wouldn’t be out of place in the Expendables lineup. Rather than turn either into a stereotype, he imbues both with a kind of emotional depth that insinuates they’re not as different as they seem.