Tonight's TV highlights

Andrew Marr's Megacities 8pm, BBC1 How do you guarantee the safety and security of citizens in conurbations of 10 million-plus souls? It is, as Andrew Marr explores via more megacity visits, tricky. In Mexico City, for example, where rich and poor co-exist uneasily, kidnapping is rife. Cue Marr learning about the "180-degree, gangster-avoiding handbrake turn" on an evasive driving course, and looking on as a bulletproof vest is put through its paces. Fascinating stuff, for all that Marr – who looks alarmingly like a demented Michael Palin in the wake of training with London riot cops – isn't a natural at this kind of meet-the-public telly. Jonathan Wright Unnatural Histories 9pm, BBC4 Despite the self-referential title, this is in fact the debut of a new natural history series from the BBC, something for which it is worth blowing out the weddings and funerals of close relatives. Unnatural Histories will explore the impact of mankind on the world's remaining wildernesses, and it starts tonight with one of the most mythologised: the Serengeti. Legions of hunters, scientists, explorers, traders and tourists have trampled all over it, mostly with no great effect – human beings are wildlife ourselves, of course – but occasionally with disastrous unintended consequence, as witnessed by a devastating virus that was unwittingly introduced by an Italian expeditionary force. Andrew Mueller The Shadow Line 9pm, BBC2 Follow the money. Ask about Counterpoint. Don't get a cat . . . DI Gabriel gets advice from Glickman (Antony Sher) as we come to The Shadow Line's penultimate episode tonight – but can he trust him (or himself?). Ratallack steps up with a bold play, Commander Penney makes some more dockside threats, and Bede is still hoping to get out. Will the box set come with a diagram? Richard Vine Mock The Week 10pm, BBC2 Dara O Briain returns to host the topical comedy show, along with regular panellists Hugh Dennis and Andy Parsons. Though supposedly a competition between two teams (though O Briain usually couldn't care less about sensible scoring), it's more a brutal contest of wit, or at least forcefulness of character, as comedians with little to say and no talent for interjecting are sidelined. This week the guests subjecting themselves to comedic natural selection are standups Milton Jones and Seann Walsh, The Thick Of It's Chris Addison and Greg Davies, AKA the sardonic Mr Gilbert from The Inbetweeners. Martin Skegg Britain And Ireland From Above 10pm, Sky 3D If we're going to have 3D telly, it's only right it should be comprised of stuff that best exploits the medium (as opposed to, say, getting up close and personal with KT Tunstall – also on Sky 3D last month). Hence this, a spectacular aerial tour of Britain and Ireland in four parts. Alternatively, you could withhold your Murdoch shilling, and just zoom in and out on Google Earth for an hour while humming along to some Vaughan Williams. Ali Catterall Ideal 10.30pm, BBC3 The most consistent sitcom currently on British screens reaches its 50th episode tonight. With Moz and Nikki now rival dealers on the same patch, they decide to unite against the looming threat of The Red Mist. Meanwhile, things get complicated over at Carmel's brothel when her ex-pimp turns up unannounced.Even after all this time, they manage to keep things fresh with the introduction of a ridiculous new enemy; a bunch of people in ginger wigs who move silently around the tower block. Julia Raeside