Keri Russell stars in a Netflix political thriller that doubles as a high-style romantic comedy.Debora Cahn most recently served as an executive producer and writer on “Homeland.” Keri Russell most famously played a hyper-efficient assassin on “The Americans.” Their collaboration in the new Netflix series “The Diplomat” — Cahn created it, Russell stars — would lead you to expect something dark, violent and complicated.But a look further back in Cahn’s history shows that she started her career with a long run as a writer and producer on “The West Wing.” And that’s the spirit she’s brought to “The Diplomat,” a political thriller laced with romance and written, with some success, in an Aaron Sorkinesque high-comic, high-velocity style.So you would be right about complicated, at least. Geopolitical crises and amorous complications are thick on the ground, intertwining and constantly morphing in ways that can be hard to follow. (The serial twists and breathless explanations both contribute to and help to obscure the plot-greasing implausibilities necessary for a show that puts earthshaking events in a comic framework.)Russell plays Kate Wyler, a career American diplomat suddenly and surprisingly named ambassador to Britain. She arrives in London with her trailing spouse, Hal (Rufus Sewell), a more experienced and renowned diplomat who is now expected to smile for the cameras but otherwise keep his mouth shut. For them, artifice is an essential element in both statecraft and marriage.Kate and Hal’s union is on its last legs, as it turns out. But thanks to an improbable, possibly MacGuffinish twist, it is mandatory that they stay together. So one pole of the plot is their highly cultivated Bickersons act, a will-they-or-won’t-they screwball anti-romance between an unforgiving woman and a roguish, egomaniacal man; you may see ghostly images of Carole Lombard and John Barrymore. At the same time, Hal, along with Kate’s fiercely competent deputy, Stuart (Ato Essandoh), is tasked with turning the combative Kate into a more refined diplomat, an arduous process with echoes of “My Fair Lady” and “Kiss Me, Kate.”The rom-com complications — they also encompass Kate’s attraction to the British foreign minister (David Gyasi) and a classic secondary romance between Stuart and the C.I.A. station chief (Ali Ahn) — and the political machinations bounce off and intensify one another, in the old “West Wing” style. An attack on a British warship in the Middle East starts an eight-episode chain of events involving Iran and Russia that has Kate shuttling between the American president (Michael McKean) and the British prime minister (Rory Kinnear, who stands out in an excellent cast) and, with Hal’s help, salvaging U.S.-U.K. relations while pretty much literally saving the world.“The Diplomat” is concerned with the dynamics of the international order, the proper balance between idealism and realpolitik, and the fallout of Russia’s invasion of Ukraine, but it’s essentially a show about a marriage. The conceit of Kate as the undiplomatic diplomat — a woman whose stone-cold, steel-trap strategic abilities would be considered suitable for the Court of St. James’s — is, to put it kindly, absurd, but its main purpose is to set up the contrast with the smoother, more devious, more obviously diplomatic Hal.And you can see how Russell’s coldblooded excellence in “The Americans” would recommend her for the part of Kate. (In an amusing nod to Russell’s long run as the murderous Elizabeth Jennings, Kate is asked whether she poisoned a fellow diplomat and deadpans, “Not my style.”) But while she’s perfectly proficient, and has no trouble conveying the character’s intelligence and, when called for, her uncertainty or anger, Russell is not as funny as the show needs her to be. Relaxing into the role and giving the emotional connections the casual, spontaneous feel that the rom-com structure calls for are not her strengths.Luckily for “The Diplomat,” Sewell has no trouble getting in touch with his inner Barrymore, and he walks away with the show. Hal is petulant, childish and arrogant, but he’s smart and charming enough to get away with it, and Sewell both embodies the charm and shows us the flashes of doubt and nobility that redeem him. Making an over-scaled, too-good-to-be-true romantic construction like Hal feel absolutely real is a trick right up there with saving the world from global war. More