DCI William Wisting, hero of the hit BBC Four series, is back in a case that will take him into the murky world of the dark web in pursuit of a killer

When the body of an Australian backpacker is found in Spain, a global group of amateur sleuths comes alive in attempting to solve the mystery of her death.

Foremost on the message boards is Astri, a young Norwegian woman whose feverish pursuit takes her closer than anyone to solving the case.

But just as she makes her claim, she goes offline.

No one in the group has heard from her since.

William Wisting is sent to investigate, plunged into a world of unorthodox methods and amateur investigation - and a dangerous case where nothing is as it seems...