Mr Grace the tailor exposes Not-Triss as a Besider changeling to her parents, and urges them to burn her. Learning she is not the real Triss, she now identifies herself as Not-Triss. The Besiders threw a stick doll containing some of Triss's possessions into the Grimmer, which emerged as a changeling with Triss's appearance and some of her memories. Pen admits she led Triss to the Grimmer to be abducted. Triss follows Pen to a meeting with the Architect. Catching it, she learns about the Underbelly where many Besiders live, and that she only has seven days to live. At home in Ellchester, she discovers a Besider delivering letters from her dead brother, Sebastian, who writes of being stuck in a perpetual winter. Her sister Pen hates her with a vengeance, and makes curious insinuations. She finds dead leaves in her hair, and her tears are spiderweb. She has a gargantuan appetite, and feels an urge to eat her possessions as well as food. Triss has been unwell since being rescued from The Grimmer, a pond near their holiday cottage in Lower Bentling. The character of Violet Parish is loosely based on her grandmother, who as a young woman shocked her home village by arriving there riding a motorcycle.

Hardinge tells us that the name of The Grimmer, the pond Triss is rescued from, is taken from the name of a millpond in her grandmother's village, Wickham Skeith in Suffolk. The Architect built it as a new home for them to move to now hidden secret places in the countryside are becoming rare. It also has a fairy underworld, called the Underbelly, populated by the Besiders, forgotten fairy beings. Ellchester is beside an estuary, hilly, and has many bridges. It is mainly set in the imaginary English city of Ellchester. The story takes place in the early 1920s, a few years after the First World War. It won the 2015 Robert Holdstock Award for best fantasy novel, and was short-listed for the 2015 Carnegie Medal.

Cuckoo Song is a children's or young adults' fantasy novel by Frances Hardinge, published on by Macmillan in the UK, and by Abrams Amulet in the USA.

Cuckoo Song by Frances Hardinge