Tha Mac an Rath Mòr an dùil gun tèid ceannas nan Cruithneach a bhuileachadh air, ach tha beachd eile aig a nàmhaid, MacRascail, a tha ag amas, le foill is faoineas, air a’ chathar a choisinn dha fhèin. Gun fhiosta do chàch, tha MacRascail an impis saorsa nan daoine aige fhèin a reic airson òr nan Ròmanach, ach ri linn ’s gu bheil Asterix is Obelix san nàbachd, is iad air a' chiad splaoid aca do dhùthaich Cheiltich eile, chan eil cùisean cho furasta dha ’s a bha e an dùil. An soirbhich le MacRascail, no am faigh Mac an Rath Mòr spionnadh bho ar càirdean gaisgeil Gallach agus taic os-nàdarra bho uisgeachan domhain Loch Ceòlmhor gus neo-eisimealas Chaledònia a ghleidheadh gu sìorraidh.

Asterix is on his first ever visit to a Celtic country outside Gaul, to the land of Caledonia – where Prince Mac An Rath Mòr is due to inherit the title of high king of the Picts. But his arch-rival, MacRascail, has other ideas and has conspired to take the high-kingship for himself. Unbeknown to the rest of the Picts, MacRascail is ready to sell out his people's freedom to the Romans, but with Asterix and Obelix at hand, and a trusted ally deep beneath the waters of Loch Ceòlmhor, will Mac An Rath Mòr come to the fore and ensure independence for Caledonia?