Hardly a week goes by without hyperbolic claims being made about chemistry between an on-screen couple. But in the fourth episode of Homeland’s first season, Carrie Mathison accidentally-on-purpose barged into Nicholas Brody and, within seconds, they couldn’t keep their eyes or their hands off each other; they couldn’t even speak without bursting into amazed, lust-filled laughter. What we had previously been led to believe was chemistry between screen couples turned out to be nothing of the sort. What was occurring between Claire Danes and Damian Lewis the second they collided: that was chemistry.

Those of us who shared growing pains with Danes in My So-Called Life were entirely unprepared for her raw-nerve performance as Carrie, the bipolar CIA operative attempting to redeem herself for failing to prevent 9/11. The mission that became her obsession was monitoring Lewis’s traumatised ex-prisoner of war Brody, a former marine suspected to have been radicalised by al-Qaida.

Carrie is regarded as a liability by the majority of the CIA; Brody is an alienated shell of a man who has more in common with his Iraqi captors than his family. Little wonder these two damaged people are so drawn to each other. Little wonder they’re so expert at lying to each other. Homeland’s first season was edge-of-your-seat electric as Carrie attempted to navigate her growing emotional attachment to Brody while racing to stop him detonating a suicide bomb.

The show’s second series was initially ablaze with intensity. There was no wheel-spinning: the CIA arrested Brody. Carrie acted as his interrogator. The CIA used him as a double agent. Homeland’s producers and writing staff then made a couple of unexpected choices. Rather than deal with Carrie and Brody as a doomed relationship built on lies, they chose to present it as a classic love affair. They also decided we needed to spend time empathising with the sulks of Brody’s teenage daughter, Dana. The young Brody was not the show’s only exasperating adolescent. The Carrie Mathison of season two became a tantrum-throwing brat, blinded by her infatuation for Brody.

By year three, everything that made Homeland exciting was gone. Multiple episodes were devoted to the decline of Brody, now exiled in Caracas, framed for setting off the bomb that wiped out most of the CIA. Weeks of watching Brody being beaten, imprisoned, becoming addicted to heroin, interspersed with visits to the psych ward where daughter Dana wound up after a suicide attempt, and the hospital where Carrie underwent electroshock therapy, grew so tedious that it came as a relief when Brody was finally put out of his misery.

But no one has, as yet, put Homeland out of the misery that has come to be its defining characteristic. Now in its sixth season, with Rupert Friend’s stoic Peter Quinn as the broken Brody substitute for Carrie to obsess over, the show has evolved into a kind of anti-24. Where Jack Bauer’s brutish attempts to defuse the geopolitical tinderbox made him a friendless global target, Carrie’s moodswings and bad decisions only seem to make her more in-demand. But there’s little pleasure in watching her save the world any more. The chemistry has gone.