Time i s a concept which is so familiar, and yet recent discoveries have revealed it to be far stranger than we could have possibly imagined. In the final programme in the series, Brian explores the enigma of time - a phenomenon we take for granted but which is one of the biggest mysteries in the universe. He recalls highlights from his TV series that touch upon this conundrum.

After four decades of searching for him, they got a one hundred percent match to the Golden State Killer's DNA. As 72-year-old former police officer Joe DeAngelo's arrest unfolds in real time, chilling facts materialize that illuminate Michelle McNamara's prescience in her book's epilogue.

Principles of generosity and prosperity inform the selection of the last two crew members: Christopher Sembroski, a former member of the U.S. Air Force who served in Iraq and now works as a Lockheed Martin engineer, and Sian Proctor, a professor of geosciences and two-time NASA astronaut candidate. Excitement for the flight ramps up, but anxiety lingers.

A fighter jet ride prepares the crew for the G-forces of spaceflight. Haley, Sian and Chris relate the emotions they experience as launch approaches.

In astronomy, we've been looking at different kinds of light and opening up the universe a little bit more of the time. But then in 2015, the roof came off. Something happened that changed everything, the ability to see waves in space and time itself. Gravitational waves help us roll back the clock to the dawn of time, discover epic cosmic collisions, on make Earth-shaking discoveries. Gravitational waves are the biggest game changer since the invention of the telescope. We have a completely new universe to view now. A new exploration of space is just beginning.

In America the summer twilight brings out a magical display of glittering lights. Millions and millions of fireflies, an ethereal sight. But behind this twinkling facade is a fierce female setting a deadly trap. The femme-fatale of fireflies is part seductress, part vampire, as Reese shows us the fireflies' deadly trickery to provide nourishment to their young.