Tha Tintin agus Dìleas an tòir air buidheann eadar-nàiseanta de mhealltairean a tha a’ dèanamh airgead fuadain. Tha iomadach rud a’ tachairt dhaibh air slighe a tha gan toirt gu ceann a tuath na h-Alba. A rèir beul-aithris na sgìre tha Biast Mhòr a’ fuireach ann an Caisteal na Beinne Mòire air an Eilean Dubh. Tha Tintin agus Dìleas a’ togail orra chun an eilein gus a’ bhiast a rannsachadh dhaibh fhèin…

Tintin and his faithful dog, Dìleas, are on a quest to track down a bunch of international money forgers. Amongst them is Dr Müller, soon to be one of Tintin’s most ruthless enemies. Escaping with his life after being shot, Tintin faces further close calls as he unearths clues which lead him to the furthermost isles of Scotland. This is where he discovers the dark legend of the Biast Mhòr monster, incarcerated within the ruins of na Beinne Mòire castle on the Black Isle. Ignoring any monstrous legends, Tintin and Dìleas take a boat to the island to challenge the legend that lies ahead.