~súile gorma cosúil leis an fharraige, gruaig dubh cosúil leis an oíche - blue eyes like the sea, black hair like the night~
Rocío Velázquez, a 16-year-old girl is growing up in the centre of Madrid in a small flat. She lives with her single mother, and younger sister Bea. Money is tight so grandmother Ana must move in with them. While helping Ana move her belongings, Rocío stumbles upon her late aunt's bedroom and along with it, an old Celtic secret.
Rocío finds it best to try and forget about her aunt's stories. However, she realises that that will not be possible when she receives an exciting letter. Soon, she discovers that she can no longer avoid her aunt's terrifying past.
Upon meeting the mer family and their supernatural enemies, a whirlwind of emotions including first love, blossom within Rocío. Rocío is challenged to accept what she wants and who she is becoming when she lives in such a judgmental environment. She must learn to make the right choices.
In the epic conclusion to the Devil's Deal series, Freya has one final chance to discover the full potential of her own power, and to use it to restore balance in the universe - once and for all. After the destruction caused by Roisin, Ferya and Torix come to terms with their losses and mistakes... And prepare for a final battle that will change everything.