As Counterpart, the Starz spy-fi drama starring J.K. Simmons, hurtles toward its season one finale on Sunday, there are several open plot threads that must be resolved. Showrunner Justin Marks also will need to set the stage for the previously announced second season of the dual universe thriller that has quickly become a critical favorite. Ahead of the season finale, here's a primer on what you need to know if you're just coming to the series now.

The Two Worlds

About 30 years ago, scientists in East Berlin were involved in a secret experiment that went wrong, resulting in the creation of a parallel Earth from that point forward. In addition to the creation of the alternate universe, a bridge between the two was opened, allowing people from either side to cross. The bridge is a closely guarded secret and is governed by a U.N. spy organization called the Office of Interchange (OI). Everyone on our Earth has an exact double, or “other,” in the parallel universe.

The big event and a major point of conflict on the series is a swine flu pandemic that decimated the alternate (Prime) Earth in 1996, killing half a billion people. Factions on Prime Earth blame our Earth (Alpha) for the pandemic and look to retaliate. A cold war has been waged silently for decades, but a plan by a fanatical Prime faction threatens to result in a full-on war of the worlds. A spy from Prime Earth is dispatched to our Earth to stop it.

Subject: Howard Silk

Howard Silk, portrayed by Simmons, is a hard-boiled spy from Prime Earth who has been crossing into Alpha Earth for years, retrieving renegade “others.” He is tasked to stop a plan mounted by “Project Indigo,” a shadowy organization headed by Alexander Pope (Stephen Rea), which has trained children from Prime Earth and turned them into sleeper agents for the purpose of replacing their doppelgangers on Alpha Earth. Howard Prime enlists his double Howard Alpha, a meek bureaucrat for OI, to cover for him on Prime Earth while he tries to thwart the attack.

The Story So Far

The main operative for Project Indigo is Clare (Nazanin Boniadi), who was trained from childhood to replace the wife of Peter Quayle (Harry Lloyd), a highly placed member of OI. She orchestrated the assassination of three members of OI in order to get the infiltrators in place for an attack on OI headquarters. Peter discovered Clare’s secret and, in a fit of guilt and anger, puts Clare in their car and drives them both into a parked car at a high rate of speed. Howard surrendered himself at OI headquarters to warn their leaders of the plot, only to be swept up in the attack. Two of the three infiltrators were killed, but one made it to the bridge between worlds, collapsing on the floor.

What Happens Next?

The third attacker is wearing a backpack, so it’s possible the goal of the attack is to blow up the portal. The OI agents didn’t follow him into the tunnel, as it has been established from early on that only one person can cross at a time.

Closing the bridge would be a game-changer going forward into season two. Both Howards would be shut out of their home universes, and each would have to find a way to exist in a strange new world.

Clare’s father Roland Fancher (Richard Schiff), an OI diplomat, got a call about the accident, so the early betting line is Clare and Peter are still alive at this point. The fallout of her betrayal and Peter’s involvement make for great drama going forward.

And is Alexander responsible for unleashing the pandemic on his own world? If so, his actions would fundamentally change the direction of the series.

The season one finale of Counterpart airs Sunday at 8 p.m. ET/PT on Starz. Click here for three theories of what could happen in the finale.