On the Oban-Mull ferry, Mull piper and council worker-cum-alcoholic, Duck slumps uninvited at the table of architecture student, Davud (a loner who has developed a fascination with fire) and a scandal-hit politician. Meanwhile, Sergeant Ivor Punch is busy policing a protest that includes a strange visitor – a man purporting to be Robert Louis Stevenson, who talks of a mysterious island that appears off Mull only twice a century, which he once saw while sailing by Skerryvore Lighthouse. As buildings burn and dead bodies are discovered, Sergeant Punch seeks to find the crux of the mystery by looking to the past. But with mysterious forces at play, he needs to decipher what is real or imagined – before the island falls into chaos.