It's 1846 and three sailors, Oliver (Alex Hassell), Jim (Graham Butler) and Cailean (Fisayo Akinade) are the only survivors of a terrible shipwreck. On their leaky lifeboat they make it to a mysterious island off the Scottish coast. Here they meet the four sole inhabitants, Douglas (Conleth Hill), his niece Lanthe (Tori Butler-Hart), the harbour man Fingal (Dickon Tyrell) and his daughter Korrigan (Alex Wilton Regan). They clearly have a terrible secret that they're hiding and when fully recovered the sailors are desperate to leave and return to the mainland, but the promised boat never appears. Oliver starts to investigate and unearths a myth of a ghostly siren that haunts the island.