Who’s going to save the world? Someone has to and, in season 4 of Netflix's scorchingly smart political drama The Diplomat, it might finally be the show’s hero Kate Wyler (Keri Russell), the American ambassador to the UK. The new run raises the stakes in terms of the fictional world events it portrays, but that’s not why it’s the best season yet: the show with the best-written dialogue on TV just got even sharper.

Dramas about politicians are obliged to be fantasies, and in The Diplomat we have two alumni of The West Wing, both more prominent this year than previously. Allison Janney’s Grace Penn has become the President, while Bradley Whitford plays her personally and professionally dissatisfied husband, Todd. But while The West Wing offered a fantasy of the American government being vastly more beneficent and conscientious than it ever has been in real life, The Diplomat is subtler, affecting the heart as much as the head.

Here, it’s vaguely assumed that everyone works within a cynical, ruthless system. What we’re invited to imagine is that clever people with good souls might still have room to score some wins. And so Kate, despite holding what in the real world is an irrelevant ceremonial post, has used her quick thinking and emotionally intelligent problem-solving to become an integral part of the US foreign policy machine.

It helps that all the important stuff keeps happening in London, and that Kate’s on/off, estranged/devoted, reliable/conniving husband Hal (Rufus Sewell) is also a political pro who has manoeuvred himself into the vice-presidency.

The season begins quietly, with our main concern being Kate and Hal’s marriage, which seemed to have righted itself last year until, just when it was too late, Kate suspected that the rapprochement might have been part of Hal’s campaign to revive his career. When protests on UK roads cause the American entourage to be diverted away from Heathrow and into an unplanned hole-up at Windsor Castle, The Diplomat’s roots as a snappy comedy of manners show through: one of its running gags is that the plain-speaking Kate, whose background is in gritty conflict resolution in the Middle East, constantly has to deal with old English traditions.

Just as it’s almost getting too sedate, though, the plot drops a bomb – not literally, no spoilers here – into the middle of the Wyler/Penn quartet and then proceeds to go, by The Diplomat’s relatively genteel standards, berserk. Large events come in torrents, leading up to an almost deliriously consequential final episode.

It is a big swing, but the show’s filigree script means the gamble pays off. Because it’s set in a world where politics is about personalities, and the most powerful people on Earth have a habit of being vulnerable eccentrics who can be read and manipulated by someone who knows what to say, when, and to whom, The Diplomat is built on two-hander dialogue. It does that better than any show ever has.

It’s dialogue in the most fundamental sense: the classic Diplomat scene has two people in conflict who test and shape each other’s view, in a tussle that may not have a clear winner but leaves both participants slightly changed. They may end up with a different idea of what their conflict actually is. And if you’re wondering whether that’s a description of a marital dispute or a globally impactful political debate, you just hit on The Diplomat’s secret sauce. It can be both.

Those scenes this season are often daringly long but all the more gripping for it and often joyfully funny into the bargain, with Kate and Hal’s endlessly fascinating battle of equals now rivalled by Grace and Todd’s delicate power imbalance. Can Kate save the world? Can she save her marriage? Frightfully clever, believably flawed people trying to work out what to say and do next to avert catastrophe feels more relevant than ever, and The Diplomat has honed it to a fine art.

The Diplomat will be released on Netflix on 15 October.

Add The Diplomat to your watchlist on the Radio Times: What to Watch app – download now for daily TV recommendations, features and more.

Check out more of our to find out what's on. For more TV recommendations and reviews, listen to .