Thorstein is the youngest son of a Norse family settled at Greenodd. Up-country is wild and unknown, but Thorstein is driven north by destiny, where he discovers the lake now called Thurstonmere. He is cheated out of his inheritance, and nearly out of his true love, but all comes well at the end. The book climaxes with a siege on Peel Island by the Saxons which he withstands. He dies valiantly, as befits a Viking, in a glorious defeat. His children and their descendants settle at Nibthwaite.
The book was a favorite of Arthur Ransome, having been on the family bookshelf. Brogan sees it also as a masterpiece of Pre-Raphaelite or Ruskinian design, and Swallows and Amazons with the theme of the exploration of the Lake as a sort of modern Thorstein of the Mere. The book displays Collingwood’s knowledge of the district, although Brogan remarks that as an antiquarian he digresses by discussing the origin of each place-name.