10-year-old Raoul of Valois battles make-believe foes, Duke Dumard and Count Jacques of Boorish Imps, by the shore of the Somme River. Upon being knocked down by his dogs, Raoul sits up and notices an odd shimmer in the shallow marsh water. It is a treasure beyond his imagination - a rusted Viking sword. Raoul raises the sword to the sky in a sign of great destiny ahead.

The next day, Raoul and his cousin return to the river to look for more Viking treasure. Their initial search yields no results until they spot a skull lodged in Viking helmet resting by dead tree, and a dagger pierced into the ground. When Raoul removes the dagger, an icy mist escapes through the mouth of the skull. The skull speaks an eerie plea for help. Its spirit is stuck and it desires freedom. Raoul and Walter stand terrified staring at the talking skull, yet both are determined to come to its aid.

After dinner, Raoul sets the skull by the hearth in their bed chamber. First, the skull asks for a bit of French wine. The boys oblige. Then the skull introduces himself as Viking seer, Cnute Fine Feathers. A hundred and fifty years ago, Cnute sailed to France with his brother, Sven Silver Hair, cousin, Harald Foul Fart, and others. They were part of a pilgrimage to pillage Paris. After being lost in a storm, Cnute's kin sailed down the wrong river - the Somme, instead of the Seine. Enraged they would never sack Paris, and gain a seat at the illustrious table in Valhalla, Sven orders Cnute to be killed for not being able to foresee their mistake. On his dying breath, Cnute curses his kin - they are to live as undead draugrs in Niflheim (world of ice and fog).by day and earth by night. The Vikings have existed in this purgatory for over a hundred and fifty years only to be seen by those who touch the Viking's belongings - Raoul's sword, the daggerx, and now Cnute's Skull. Upon Unfortunately for Cnute, he too was cursed. His spirit can only be freed from his skull when his brother's curse is lifted.

Raoul, Walter and their friend, Gunhilda, daughter of a nearby Norman farmer seek to spy on the Viking draugrs at night. After returning across the River in Gunhilda's boat, the boys watch the Vikings swim across finding Cnute's skull missing. They read an inscription carved the base of the dead tree: To break their curse they must kill a French nobleman. Raoul and Walter realize in order to save themselves and their family, they must stop the cursed Vikings.

The next day, the boys visit Gunhilda's family and are introduced to her Poppy who reminds Raoul as a real-life Viking. They are inspired to erect a nithing pole on the Viking's island. The children craft a fake horse head out of a hemp sack and straw. Upon planting it on the island, Cnute teaches the children a chant that will set the intention to place a mystical boundary around the island to keep the Vikings from leaving. At night, the children return to find an icy mist hovering around the island. When one of the cursed Viking steps into the mist, he is enveloped into a fog and disappear with an otherworldly scream. The curse worked. The Vikings are bound by o the island and Raoul's family is safe...for now.