It sounds like a Hollywood blockbuster: a deadly asteroid is on a collision course with Earth. But in reality, it's only a matter of time before a giant space rock threatens to wipe out civilization. An asteroid took out the dinosaurs sixty-five million years ago. Are we next? This episode analyzes the threat and explores the many ways--from a nuclear bomb to ingenious new technology--that experts are proposing to stop Armageddon

Three years after the devastating fire, Lucy Worsley has exclusive access to the Cathedral of Notre-Dame de Paris at a key turning point of the restoration. Work to rebuild and restore the monument is being ramped up in a bid to meet the ambitious deadline to reopen it to the public in 2024. Scaffolding now fills the interior of the cathedral, giving Lucy and the team unprecedented close-up access to every inch of the structure. Lucy meets scientists, historians and craftspeople working to return the 850-year-old Gothic masterpiece to its former glory.
The fire coated Notre-Dame with tons of toxic lead dust, so specialists are now decontaminating the site. Unique access to the stunning rose windows allows Lucy and the stained-glass historians to uncover the mysteries of these enigmatic 13th-century works of art. An unforeseen legacy of the fire threatens to destroy the remaining vaulting, so the team must develop an ingenious solution to save the stonework. The loss of the roof during the fire reveals hundreds of large iron staples embedded in the stone along the top of the walls. And the incredible task of sourcing and cutting two-thousand perfect oaks gets underway to rebuild the world's most complex medieval timber structure - Notre-Dame's extraordinary roof and spire.

Ewan and Charley make their way to Peru from Bolivia, and the scenery is just beautiful. Ewan wanted to visit Machu Picchu since he was a kid and decides to show the world the Incan citadel set high in the Andes Mountains. Next up, they make their way to the Cutivireni, a community of Ashaninka people in the Amazon rainforest, with its carbon program to limit the magnitude or rate of global warming and its related effects.

Black Holes are one of the most destructive forces in the universe, capable of tearing a planet apart and swallowing an entire star. Yet scientists now believe they could hold the key to answering the ultimate question - what was there before the Big Bang? The trouble is that researching them is next to impossible. Black holes are by definition invisible and there's no scientific theory able to explain them. Meet the astronomers attempting to image a black hole for the very first time and the theoretical physicists getting ever closer to unlocking their mysteries. It's a story that takes us into the heart of a black hole and to the very edge of what we think we know about the universe.

Zachary Quinto sets out to research the strangest creatures dug up from the depths of the ocean and to see how much they lend to the monsters that are depicted in myth and legend. Starting in Australia, he meets with a teenager who was savagely attacked by a swarm of mysterious flesh-eating monsters, only to jump into the water himself the next day amongst highly venomous sea creatures.
From the carnivorous fish of American rivers to the eyeless monsters of the Atlantic Ocean, Zachary finds some merit in these old monster stories and is starting to understand just what we mean when we say that we know less than 1% of what waits in the depths below.

For Charley Boorman and Ewan McGregor, their journey in the smooth Ecuadorian roads would most definitely have come as a relief after some rough terrain riding. Next up, they stop by one of the factories that make the famed Panama hats. One of the local producers explains that making them by hand takes roughly up to a month. At the other end, the crew on the Rivians make a dash for Central America where they have to ferry their trucks across as the Darién Gap blocks their path. On their way, they make a quick pitstop at one of the cacao factories to know more about cocoa production, one of the economic drivers of the country.