For three seasons now, The Americans has dodged and weaved around the matter of its characters’ integrity. While the show remains as complex and nuanced as ever, the new season of the FX drama begins with a hint of serious escalation for both its central couple and viewers. In Wednesday night’s premiere, an informant hands a biological weapon to the KGB sleeper agents Philip (Matthew Rhys) and Elizabeth (Keri Russell), and the horror on their faces may give answer to the question the show has asked all along: How far is too far to go in pursuit of a Cold War victory?

Already, Philip and Elizabeth have crammed the corpses of innocent bystanders into suitcases, engaged in coercive romantic relationships with teenagers, and created a sham marriage with a State Department employee that’s lasted years. Through it all, The Americans has stayed compelling because it lets the impact of every horrible deed hit the audience as hard as it hits its protagonists. But now, its heroes are slowly transforming from old-fashioned spies into out-and-out terrorists, and in the first few episodes of season four, they seem ready to crack. The audacious gamble at hand centers around how long viewers can stick with the Jenningses before they crack, too.

Philip and Elizabeth’s union, arranged years ago by the KGB, has always offered a twist on the stereotypical ’80s nuclear family, where a husband and wife married too young, got wedged into a dull suburban routine, and stayed together for the sake of their kids. Last season, their loyalties and their union were tested as the Soviet Union demanded they groom their daughter, Paige (Holly Taylor), as a next-generation sleeper agent—an idea that appealed more to Elizabeth (who desires to share more with her daughter) than Philip (who wants for her to have a normal life). Paige now understands that her parents are spies, but has only been given a sanitized version of what they do. The tension over this deception ratchets up quickly this season as the family’s internal battles continue.

Meanwhile, the Cold War intensifies, with grave consequences for most characters. The biological weapon given to the spies by a disgruntled informant played by the great character actor Dylan Baker (who’s long excelled at portraying grumpy grey functionaries nursing dark secrets) suggests that the KGB’s tactics will grow increasingly desperate as the Iron Curtain begins to crumble. This isn’t just plot context: It raises the stakes of the battles in Philip and Elizabeth’s marriage. Even if Paige isn’t in the hands of Soviet monolith that brought them to the United States, she will doubtless face a very uncertain future.

That uncertainty is what drives the fourth season to be as consistently compelling as the previous three. Viewers presumably won’t wonder about what will happen to the Berlin Wall or Mikhail Gorbachev so much as whether Philip and Elizabeth can throw off the weight of their evil acts, or at least help their children escape them. The fourth season premiere, “Glanders” (named after the deadly virus handed over by Baker’s character), begins with a flashback to Philip’s youth and the first murder he committed, a memory he’s trying to unburden himself of. He still attends the Est self-help seminar (one of the many ’80s fads that litters the show’s margins) to try and deal with his guilt, but is unable to discuss his sins in public. He also lets his sham wife Martha (Alison Wright) learn more about his deception than she ever has before, a decision that should have major repercussions.

The virus Philip has to hold onto makes for a powerful, if obvious, metaphor for the greater evil he’s burdened with. As the nations he’s caught between escalate their tactics, he and his victims pay the human cost. Meanwhile, Paige is asking more and more questions about what her parents do at night; Elizabeth replies by insisting that her work revolves around getting people to trust her in order to help build a more cooperative future in a world racked with conflict. The tidy, idealistic picture is clearly meant to appeal to a teenager who campaigns for nuclear disarmament and follows a friendly youth pastor preaching for world peace. The audience knows the truth, but The Americans’s showrunner, Joe Weisberg, mines exquisite drama from the intricacies of each lie being told, as all of the show’s alliances continue to teeter on the brink of disaster.

It feels almost futile to advocate for new viewers to jump on board with The Americans, which has drawn consistently mediocre ratings on FX but equally consistent rapturous praise from critics. The show still feels like the closest heir to Mad Men and Breaking Bad, the last great consensus prestige dramas on television, but it may not be discovered widely until after it’s gone and exists only on some streaming website (its earlier seasons can be viewed right now on Amazon Prime). Weisberg has long since stopped caring about catching newcomers up, and instead seems ready to start barreling towards a conclusion. We know what will happen to the Soviet Union, but for the show’s remaining fans, Philip and Elizabeth’s strained union is the reason to stay on board. This year it will be tested in unimaginable ways.