We are in the grip of an allergy epidemic. 50 years ago one in 30 were affected, but in Britain today it is closer to one in three. Why this should be is one of modern medicine's greatest puzzles. In search of answers, Horizon travels round the globe, from the remotest inhabited island to the polluted centres of California and the UK. We meet sufferers and the scientists who have dedicated their lives trying to answer the mystery of why we are becoming allergic to our world.

Professor Robert Bartlett explores how the Normans developed from a band of marauding Vikings into the formidable warriors who conquered England in 1066. He tells how the Normans established their new province of Normandy -'land of the northmen' - in northern France. Under the leadership of Duke William, they conquered England. The Battle of Hastings marked the end of the Anglo-Saxon aristocracy and monarchy. The culture and politics of England would now be transformed by the Normans.

Professor Jim Al-Khalili sets out to discover whether nuclear power is safe. He begins in Japan, at the former Fukushima nuclear plant, where he meets some of the tens of thousands of people who have been evacuated from the exclusion zone. He travels to an abandoned village just outside the zone to witness a nuclear clean-up operation. Jim draws on the latest scientific findings from Japan and from the previous explosion at Chernobyl to understand how dangerous the release of radiation is likely to be and what that means for our trust in nuclear power.

Playing with Nuclear Fire' - In March 2011, the Tohoku earthquake in Japan created a tsunami that killed some 16,000 people and crippled the Fukushima Daiichi nuclear power plant. In the aftermath, the government and TEPCO, the plant's operator, withheld information about the extent of the damage. Three years later, citizens and the international community are left wondering if Japan really does have the situation under control, as the government is insisting, or if the danger is far greater than anyone is willing to admit. 'No Man Left Behind' - It's estimated that over a quarter million vets from recent wars have sought treatment for PTSD. Despite these statistics, veterans often face an uphill battle to get treatment, receiving inadequate attention and, most dangerously, overprescribed narcotics and other pharmaceuticals. Ryan Duffy meets veterans struggling with mental illness and addiction.

For 45 years, America was locked in the Cold War with the Soviet Union, and fear of global nuclear annihilation was constant. The end of the Cold War in 1991 was supposed to usher in a new era of peace and cooperation, but it didn’t last. Tensions between the U.S. and Russia have been simmering for years. And now, the conflict in Ukraine has pushed the relationship to the brink of full-blown crisis. VICE Founder Shane Smith met Kremlin officials and American leaders to figure out what’s really driving the new standoff between the powers, while correspondent Simon Ostrovsky reported from the front lines of the bloody war in Eastern Ukraine.

During a Gurkha recruitment Palin is disturbed by Maoist insurgents, but survives to suffer as he climbs to 15,000 feet and sees the majesty of Annapurna Sanctuary. In Kathmandu he is blessed by the Nepalese king before meeting some holy men. Crossing into Tibet, he meets his first yaks at the highest monastery in the world, before heading up the Rongbuk glacier towards the summit of Everest.