This episode examines the idea of scientific racism, an ideology invented during the 19th century that drew on now discredited practices such as phrenology and provided an ideological justification for racism and slavery. The episode shows how these theories ultimately led to eugenics and Nazi racial policies of the master race.

1917. People have had enough of the war. Behind the scenes, uprisings are brewing, like the one that toppled the Tsar in Russia; on the front soldiers begin to mutiny as they did at Chemin des Dames in France. German submarine attacks in the Atlantic will finally pull the Americans into the war, but they arrive too late to help prevent the carnage of the Battle of Passchendaele.

Mid-1942, as Hitler’s forces are pushing into southern Russia to take the oilfields, he spies a city prized by his enemy Stalin – Stalingrad. If Hitler can capture this city, he can expand his empire all the way to the Urals. But what Hitler hasn’t counted on is the enormous resilience of the Soviet people; men and women willing to defend their Motherland at all costs.
What ensues at Stalingrad is one of the bloodiest battles in the history of warfare, with an estimated 2 million total casualties, and an event that turns the tide for the Germans.

Follows the epic operation to secure, raise, and salvage the Costa Concordia cruise ship, which ran aground and tragically capsized off the coast of Italy on January 13th 2012, killing 32 people. The wreck stretches the length of three football fields, weighs 45,000 tons, and lies half submerged on the site of a protected reef, with a 160-foot-long hole in its hull. Moving it from its precarious perch on the edge of an underwater cliff will be a huge technical and logistical challenge. Joins a team of more than 500 divers and engineers working around the clock as they attempt the biggest ship recovery project in history.

Dr Hannah Fry travels down the fastest zip wire in the world to learn more about Newton's ideas on gravity. His discoveries revealed the movement of the planets was regular and predictable. James Clerk Maxwell unified the ideas of electricity and magnetism, and explained what light was. As if that wasn't enough, he also predicted the existence of radio waves. His tools of the trade were nothing more than pure mathematics. All strong evidence for maths being discovered.
But in the 19th century, maths is turned on its head when new types of geometry are invented. No longer is the kind of geometry we learned in school the final say on the subject. If maths is more like a game, albeit a complicated one, where we can change the rules, surely this points to maths being something we invent - a product of the human mind. To try and answer this question, Hannah travels to Halle in Germany on the trail of perhaps one of the greatest mathematicians of the 20th century, Georg Cantor. He showed that infinity, far from being infinitely big, actually comes in different sizes, some bigger than others. This increasingly weird world is feeling more and more like something we've invented. But if that's the case, why is maths so uncannily good at predicting the world around us? Invented or discovered, this question just got a lot harder to answer.

Tim Whewell is in St Petersburg to meet the self-styled Russian nationalists and patriots who are volunteering to join the fighting in eastern Ukraine. He joins a group of volunteers as they undertake military training, and travels to Ukraine to see how the volunteer force is fighting on the ground.