The Weekend's Television: Wallander, Sun, BBC1Louis Theroux: Law and disorder in Philadelphia, Sun, BBC1After Rome: Holy War and Conquest, Sat BBC2

Take a cold, hard look

The titular detective in Wallander is gloomy – a crapulous, grey-looking figure in Ken Branagh's characterisation, poised unhappily between hangover and nervous breakdown.

Wallander's surroundings are anything but gloomy, though. Should he want to look on the bright side, he only has to lift his head, because this BBC adaptation of Henning Mankell's Swedish thrillers is as vibrantly colourful as anything we've seen on screen for months. The mood is noir, darkened by corruption and human cruelty and compromised justice, but the look is primary-colour picture book, gleaming with northern light. The opening scene gave a fair sense of the combination: in a field of glowing yellow rape, a terrified girl panicked at Wallander's approach, poured petrol over her head, and ignited a great marigold blossom of flame.

This didn't exactly improve Wallander's mood, and, as I've said, he's not exactly one of God's little sunbeams to start with. He's separated from his wife and living alone in bachelor squalor. His daughter is exasperated with him, his distant artist father is showing the early signs of Alzheimer's, and wherever Wallander looks he sees evidence of social decay. He's testy, too. When a brash young policeman in the office suggested that the fatal hatcheting of a prominent former politician might take precedence over the suicide of an unknown teenager, Wallander snapped back: "Fifteen-year-old girl burns herself to death and you don't think that's a crime?" If society turns out to be to blame, Wallander wants it in the dock.

All but the most half-witted viewer would already have guessed that the two crimes are connected anyway, and as a serial killer with a penchant for scalping his victims worked his way through Swedish high society, that link was gradually revealed. Sitting waiting for it to emerge was often a visually dazzling experience, the camerawork as attentive to the contours of Branagh's stubbly, despairing face as it was to the Swedish locations in which the action took place or the bruised pastels of a Munch sunset. But it couldn't really be said to be thrilling or unexpected. Branagh's playing of Wallander is utterly heartfelt, but the character oddly feels shallower than the performance, the disaffection and Weltschmerz just another detective gimmick.

That real policemen do get gloomy is undeniable. There were a lot of jaded and melancholy cops in Louis Theroux: Law and Disorder in Philadelphia, understandably depressed by the fact that the local citizenry would rather die than offer them assistance, but will then also berate them for their failure to control crime. Fans of The Wire will have found themselves in deliciously familiar territory here, in a film full of bantering cops and swaggering corner boys, row houses and vacant lots, casual drug murders and a sudden, universal myopia when a crime has been committed and a witness is called for. The only distinctive feature was the gangly rubber-necker in the body armour, asking gauche questions. "Is she a relative?" he asked tentatively about a weeping girl at the scene of a fatal shooting. "She" was the dead man's sister, and Theroux's nudging curiosity about her grief felt distinctly uncomfortable here.

It was a fascinating film, though, in part because of the utter dereliction of this particular patch of the inner city, but also because it confirmed that The Wire had, if anything, understated its portrait of civil breakdown. At one point, Theroux went to talk to a local drug baron called Reds, a nearly spherical figure draped with bling, who insisted, with a lazy, amused assurance, that he earned his living by trading in real estate and used cars. Reds also ran his own equivalent of an inner-city farm: a calf, a goat and a sheep that were let out to graze on a vacant lot. "We don't have the acres, you know?" Reds said apologetically, as the cow nibbled its way around the crack vials and litter. You wouldn't have dared make it up, but there it was, a little dream of another life beyond the chain-link fences.

After Rome: Holy War and Conquest gave us Boris of Arabia, London's Mayor lilting over the brow of a sand dune on the back of a camel, five inches of pallid shin between his black socks and rucked-up trousers. Boris was in Palmyra in Syria to kick off a two-part film about the grating, sometimes violent history of the relationship between Christianity and Islam, a topic he pursued from Jerusalem – "the San Andreas fault" of the clash of civilisations – to Spain, where the average bloke in the street is still inclined to downplay the part played by 800 years of Islamic rule, as evidenced by several average blokes who insisted that the Visigoths had been far more important. Johnson is a lively presenter – as likely to quote Pulp Fiction as he is to cite Edward Gibbon – and the subject is an important one. As Johnson reminded us, if you wanted to find a jihad in the 11th century, it was the Christians you'd need to turn to.