Meanwhile, in a seemingly unrelated plotline (which of course means it’s not), former British soldier Luke Gardner (Lennie James, “Jericho”) is on death row for murdering two girls. Trying to save him is Jane Lavery (Eva Birthistle, “Silent Witness”), a government human rights lawyer.

The performances are uniformly solid. Daniels makes for an appropriately menacing MI6 man. And James, as a man marked for death in mere days, makes Luke a prickly mix of bitterness and tortured loyalty. James refuses to let Luke sink into saintliness.

But the real star of the series is Isaacs. He’s proven he can play dark characters, like the vicious, small-time Boston mobster in the Showtime series “Brotherhood.”

But here he does something very different. His Mark Brydon is an ambitious man, a rising political star, who over the years has let expediency replace conscience as he moved up in the diplomatic corps.

Only now, in a moment of personal and political crisis, is he rediscovering that conscience.

Isaacs makes that transition convincing, even poignant, especially amid all the chaos of the larger story.

That chaos, apart from occasional preachiness, is actually the biggest weakness of “The State Within.” Even over six hours, the plot often gets irretrievably convoluted, to the point where trying to keep up becomes fruitless. Luckily, the plot details aren’t as important as how the characters react to them.

If “State” has another weakness, it’s the one-dimensional way in which some of its American characters are portrayed. They’re not stupid by any means but few of them are allowed the emotional complexity of the Brits. Sharon Gless’ Defense Secretary Warner in particular seems primarily defined by the combat death of her son.

Despite those problems, “The State Within” works most of the time, maybe not as a BBC take on “24” but as a deeper study of how terrorism still affects our lives and as a portrait of a man struggling to rediscover his idealism. In that it’s spot on.