Karl Pilkington tries to find out how people achieve happiness, something he isn't sure it is worth pursuing. In Mexico, he joins members of a tribe who enjoy running ultra-marathons and meets a group that finds pleasure in pain. In LA he hangs out with some hip-hop clowns, experiences a day of beauty with a plastic-surgery addict and gives up his possessions to see if the simple life is the answer to contentment.

Why do people have children? That's the puzzler bothering world-weary philosopher Karl this week. Travelling the globe, he tries to understand why couples feel the need to become parents, visiting a fertility festival in Japan and a natural birthing centre in Bali - where he lends a hand with a delivery - and meets an LA couple looking to take the next step.

Karl aims to get to the bottom of his relative lack of a vocation, and visits an 85-year-old Japanese inventor to discover whether he has what it takes to become a genius. When the results prove less than encouraging, he sets his targets a little lower by trying his hand at becoming a traditional Japanese handyman. On the opposite end of the spectrum, he hangs out with a self-made millionaire in South Africa and jets off to Los Angeles to strut his stuff in a Hollywood fashion show.Karl makes a favourable impression on brazenly ostentatious South African playboy Kenny. Kenny's the kind of guy who sits on a throne at home and is keen to show off his wealth to the cameras, so Karl receives the full rap-star treatment. He sips champagne by the pool, rides in a helicopter and gets a facial. And then moans about it all.