It’s a hot summer’s day in 1998. Two couples stand side by side in a small courtyard. Brought together by chance, they may never have met if not for one thing. A tatty piece of paper, the contents of which will change their lives, and the lives of thousands across the country, forever.

Hannah Fry tells the tale of this single sheet of A4 uncovered a dark pattern and a serial murderer hiding in plain sight.

A warning for sensitive listeners, that this episode contains references to murder.

Episode Producer: Lauren Armstrong-CarterSound Design: Jon Nicholls Story Editor: John Yorke

A series for Radio 4 by BBC Science in Cardiff.