NEARLY 16m Americans tuned in to watch the finale of the fifth season of "The Walking Dead" on AMC on Sunday night, giving the show higher ratings in the key 18-49 demographic than mainstream shows such as "The Voice" or "The Big Bang Theory". Non-viewers may see this as a sign of an unhealthy interest in the blood and gore that are the inevitable accompaniment of a show based on a zombie apocalypse and, indeed, the ingeniously vivid special effects no doubt draw in many viewers. But the show's success is based on a lot more than that. The creators quickly recognised that there is only so much that can be done with unspeaking, slow-moving zombies. They are the background to the plot in the same way that nuclear war is the background to Cormac McCarthy's "The Road". What matters is how humans react in the face of this disaster.

The series is built around a small (and ever-changing) group of survivors, led by ex-policeman Rick Grimes, played by the British actor Andrew Lincoln. Only four other cast members have been ever-present: Rick's son Carl; Glenn, an ex-pizza delivery man and the conscience of the group; Carol, who started the series as an abused wife; and Daryl, the crossbow-wielding, motorbike-riding hillbilly who is the favourite of many fans. A good deal of the suspense stems from the fact that fans cannot be sure any character is entirely safe; a zombie or human attack can carry them off at any moment. Rick has lost his wife and his best friend, Carol her daughter and Daryl his (admittedly psychotic) brother.

The basic premise is that civilisation has resorted to the Hobbesian description of life as "nasty, brutish and short". The old world has gone for good; one episode explicitly suggested that the survivors themselves were the "walking dead", blundering from one situation to another while merely postponing their inevitable demise. Other groups have survived by resorting to cannibalism or brutish dictatorship. Any attempt to settle down (the group occupied a prison in series 3) is doomed by the arrival of predatory outsiders. That looks likely to be the theme of series 6, with the group finding supposed sanctuary in a suburb outside Washington, run by a former Congresswoman. But a marauding band called "the Wolves" is not far away.

Not every episode works, as the show's creators (who have gone through the odd culling as well) balance the need for character development with that for action; a few zombies must be dispatched every time. But at its best, the show has the ability to make viewers care about its characters (Carol has developed from milquetoast into a modern-day equivalent of the Ellen Ripley of the "Alien" films), to deliver genuine shocks and to raise some decent moral dilemmas. The season finale almost saw Rick expelled from his suburb, only for the residents to realise how much they depend on his capacity for violence and quick-thinking.

The show has been successful enough to generate a spin-off, set in Los Angeles, that will air this summer. And there is no obvious reason why the original show should end soon. Just as long as the writers don't kill Daryl.