On a picturesque island in the sea, little boy Ben (voiced by David Raul David Rawle) and pregnant mother Brona (voiced by Lisa Hannigan Lisa Hannigan) spent their last happy and unforgettable night. Soon after, sister Sirsa (voiced by Lucy O'Connell Lucy O'Connell) was born, but died because of her mother. Six years later, their father Connor (voiced by Brendan Gleason Brendan Gleeson) never got over the loss of his wife, while the fantasy Sylsa stumbled upon Beidi left by her mother and nearly drowned in the sea, leading Grandma to forcibly take the two children off the island. Brother and sister can not adapt to the life of the city, they are walking in the street, accidentally encountered three strange elves. They hope to return to their hometown with the help of the song of "Seal Girl" Sisa, and the magical adventure begins. This film is based on Irish folklore.