The Sea Thy Mistress is the third and the final book in the "Edda of Burdens" trilogy. It continues the story of the characters introduced in the first book, All the Windwracked Stars, and the second book, By the Mountain Bound.

Muire, the last walcyrige, sacrificed herself in the first book to prevent Ragnarök, the end of the world. She took the place of the Bearer of Burdens, a mythical wyrm in the sea. In doing so, she rejuvenated the world, made the soil fertile again and brought back the plants and animals which had died because of manmade radiation and poisons. People are settling the countryside again and getting on with their lives. Most of them honor Muire as an angel and know what she sacrificed for them.

However, Muire's lover, Cathoair, is a tortured soul. He comes from an abusive home, has lost both of the women he loved, and blames Muire bitterly for leaving him. He has been changed into an immortal angel and he roams the country helping people. Then he interrupts a gang raping a woman, rescues the woman, and tries to build a new life for them both.

Thirty-six years after Muire went to the sea, she gives birth to a son who washes up on the beach. When Cathoair finds out that he and Muire have a son, he leaves his new life to rear his son, Cathmar. But an old enemy from the world before the apocalypse has returned. Thwated in her attempt to instigate the end of the world, Heythe will just have to start again.

Most of the characters from All the Windwracked Stars return. The old cyborg Aethelred is now a priest of Muire. He keeps the temple overlooking the sea and he finds the boy on the beach. The moreau, the human/animal hybrids, are free of their former slavery to the Technomancer and are working as ordinary people, some as peacekeepers among the humans. Selene, the snow leopard hybrid from the previous book, is one of latter. She keeps the peace and also maintains contact with Aethelred, Cathoair, and the Grey Wolf.

Mingan the Gray Wolf, the oldest character, survived the war among angels and later the apocalypse caused by humans. The Sun Eater is a captive inside him. He used to be more wolf-like and acted on instinct alone but has since learned to think more before acting. When he discovers that the ancient and powerful goddess Heythe has returned, Mingan decides to move subtly against her. He already has three allies--Selene, Aethelred, and his magical steed Kasimir--and an ancient weapon: the Imogen, a soulless creature who feeds on human misery.

Author Elizabeth Bear focuses here on relationships. Cathoair's father beat him, and Cathoair is determined to be a good father to Cathmar. Aethelred is a wise confessor and advisor to Cathoair, but he roams the world helping people, so he is not around a lot. Cathoair loathes Mingan and wants nothing to do with him. Through the course of the book, Cathmar grows into the difficult teen years. An immortal angel tasked with helping other people, he knows his mother only through the legends of Muire. His youth and inquisitive nature provide quite a contrast to the rest of the cast. There is also a great contrast between the rebirthing world, full of new growth, and most of the characters. Some carry a burden of guilt and shame, blaming themselves for Muire's sacrifice, and most carry bitter memories. Cathoair is especially depressed, although he keeps that a secret from his son who is so like the new world, young and eager to learn new things.

Bear grounds the novel in Norse mythology but uses the ancient tales in way that make them unique to her; the mythology is a starting point, and this trilogy builds upon it rather than simply rehashing old myths.

The pace of this book is slower than its predessors, its mood different. All the Windwracked Stars was set into an apocalyptic world with desperate stakes; in By the Mountain Bound, a prequel to the first book, the einherjar and walcyrgie were struggling against their inevitable fate. In The Sea Thy Mistress, things have turned for the better, and the characters strive for their individual redemptions and healing, as well as saving the world. All in all a very satisfying conclusion to the series.