What sort of alien civilizations might exist in the vastness of space? Terra is the fictional world imagined in Episode 4, a planet nine billion years old, twice as old as Earth. Old enough that a truly advanced intelligence could evolve. It was once a fertile world, now it is barren. But life can still thrive here in artificial domes. Over time, they've evolved not to need their bodies. They exist only as neural tissue. They never age, they never die. They're monitored and maintained by robots.
If alien civilizations are statistically so likely, why haven't astronomers found any sign of them? Where is everyone? Every time we look at an individual star, that's like dropping a bucket in the ocean. We're going to have to look at a lot of stars and to search through a lot of data until we find the clue that leads us to another civilization.

Each night, across the planet, our cities illuminate the darkness. Their lights burning brighter than ever before. And whilst we're asleep, animals are making the most of this new neon landscape. The sunset in Los Angeles, Chicago, and Toronto creates worlds of opportunities for the cleverest animals willing to take the risk.

Sunset over the plains of Kenya's Maasai Mara, one of Africa's most protected wild spaces. And home to the fastest land animal on Earth, the cheetah. These agile cats have always been thought of as daylight hunters. They target prey from more than a kilometre away and chase it down at over 100 kilometres an hour. In one of these plains, two cheetah brothers attempt a high-speed night hunt with a gang of powerful hyenas on their trail.

Global CO2 emissions coming from aviation will go from about 2% to, by 2050, something closer to 20%. Technology is the only key to open up a brighter future that can be sustainable at the same time. The way we fly is about to change, driven by a new breed of aviators not afraid to think differently. From clean, green electric aircraft to autonomous sky taxis, could the days of the jet age be numbered?

It is the greatest technological challenge ever undertaken by humankind. The quest to produce the ultimate energy solution, Fusion, a reliable source of clean power. What if we could create here on earth, our very own version of a star, a machine that makes power from nuclear fuel? But so immense are the challenges, the goal of a power-producing fusion reactor has so far remained elusive and out of reach... until now.

On the second part of his journey through the dark ages Richard Rudgley continues into the age of the wandering peoples, the Volkerwanderung. These Northern people enjoyed a golden age unaffected by Rome and just 30 years after the Romans relinquished Britain, the 'Anglo-Saxons' made their move. The bedraggled legions are in retreat. Walls are pulled down. Mosaics shattered. And yet there never was a people called Anglo-Saxon. We look at the lasting influence of Saxon leaders like Alfred the Great, and his blue print for social justice.