Leah Washington and her new boyfriend Joe Pugh are on their first day out together. They're at Alton Towers theme park, where they've chosen to ride the "Smiler" rollercoaster: a terrifying tangle of track that loops and swoops through a world-record 14 inversions. Leah and Joe are seated right at the front of the train and, as they reach the highest point of the ride, they steel themselves for the drop. But then, quite suddenly, the ride stops.

Down on the ground, the computer system that controls the rollercoaster is warning that another carriage is out on the track, right in the path of Leah and Joe's train. The engineers are certain the computer is wrong...

Algorithms are often faster and cleverer than humans, and they can help us avoid accidents. But computers can make mistakes. When should we trust our own heads, instead of the machine?

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