Louis Theroux: Law and Disorder in Johannesburg BBC2, time

Speccy uber-geek Louis Theroux's long-practised faux naif act may be looking threadbare in certain journalistic circs, but it proved its worth when deployed on the new South Africa's most blood-stained, poverty-stricken thoroughfares.

Theroux had naturally chosen to arm himself against trouble with a wide-eyed look of incredulity and an impulse to ask questions so obvious that even a six-year-old would think twice about voicing them. "Guns?" he squeaked at one point, interviewing a dead-eyed dude on a blasted Jo'burg street corner.

"Of course - I got one here," the dude replied, uncovering a revolver, tempering his menace with a smirk. Louis's disingenuous inquiries prompted the same looks of disbelief, mild disdain and wry amusement from most locals he met: the bad guys, the slightly-less-bad good guys and the desperate-to-the-point-of- murderous guys in between.

No-one took Theroux entirely seriously, or feared him, and so no-one thought twice about telling him all. The outcome was a programme that provided illumination in the same measure that it made you weep for the future of humanity in Africa.

In the alarming process of assembling Louis Theroux: Law and Disorder in Johannesburg, TV's premier nerd-savant probed the emergence of privatised South African police forces. This he did by joining some of these mini-armies as they patrolled what must be the urbanised world's deadliest, most nightmarish neighbourhoods.

Bad Boyz Security ranged around inner-city, high-rise Hillbrow, brusquely evicting tenants from the area's hi-jacked apartment buildings, readily transformed into teeming slums by rapacious landlords. Bad Boyz were charismatic and polite bullies, mostly keeping their Parabellum Lugers tucked away, gangsta-style, and snickering only modestly about the deaths of those who oppose them.

In the outlying shanty town of Diepsloot, Theroux listened as paternal Walter, the sad-eyed old boss of security firm Mapogo, explained how to give some medicine or a nice cooking - briskly administered by leather whip, or sjambok - to any errant skellum you found.

Theroux didn't need to ask what a skellum was, as one was sitting nearby in an armchair, bloodstains down his shirt, his arms pinioned behind him. Correcting himself, Walter further explained that he hadn't meant to say skellum, he'd meant suspect.

Everyone is judge and jury in Diepsloot, as Walter himself found out not long afterwards, needing SA's rarely-seen official police to rescue him from a vigilante mob who felt he'd nicely cooked/given medicine to - with a golf club - the incorrect skellum (sorry, suspect). Theroux hovered, innocently asking the mob what form of instant justice they sought to impose on Walter.

"We want to burn him," they replied, with terrifying calm. Poor Walter sat in the back of a police van, relatively safe - but not from Theroux's naive questions. Despite facing summary death, Walter gamely answered them all. The mob dispersed, possibly disarmed by Theroux's winning brand of assume-nothing/ask-everything journalism.

Later, Theroux met a real bad baddie, malevolent Maleven, growling in a gravel-voiced slur as clouds of smoke billowed from his mouth and nostrils. The fumes could have been nicotine - or just as easily sulphur from the bowels of Hades.

Maleven's horrifying boasts of criminality led Theroux, deploying his naive schtick once more, to wonder whether he'd been joking. His stunned local guide almost fainted at the notion, thereby confirming how truly lawless South Africa is, just as Theroux intended.