Aleks Krotoski tells the story of a film that doesn't exist and the online community convinced that it does.

We hear from people who have come together on the online site Reddit to share their memories of the film, including a former video shop worker called Don.

Many of them have very clear memories of watching Shazaam and are convinced it's disappearance is related to a strange phenomenon called The Mandela Effect, so named after the late South African activist Nelson Mandela.

We follow Don on an epic journey as he tries to uncover proof. Along the way we'll encounter conspiracy theories, alternate worlds, computer simulations and a recently deceased Australian inventor called Henry Hoke. It's going to get weird.

But what does this willingness to believe in something despite all evidence to the contrary tell us about the online world and the way communities form in the digital sphere?

Aleks speaks with anthropologist Genevieve Bell about the stories we tell; cognitive scientist Donald Hoffman and Professor of Philosophy at Oxford University Nick Bostrom. Amelia Tait of the New Statesman explains how the story of Shazaam has evolved online.

Producer: Caitlin Smith.