On a picturesque island in the sea, the little boy Ben (voiced by David Rawle) and his pregnant mother Brona (voiced by Lisa Hannigan) spent the last happy and unforgettable night. Soon after, his sister Saoirse (voiced by Lucy O'Connell) was born, but his mother passed away. Six years later, their father Connor (voiced by Brendan Gleeson) has never gotten out of the pain of losing his wife, and the fantasy Saoirse accidentally discovered the bead flute left by her mother, and almost drowned in the sea, which eventually led to her grandmother forcibly taking the two children away from the island. The brother and sister could not adapt to the life in the city. When they were wandering in the streets, they accidentally encountered three strange elves. They hope to return to their hometown with the help of the singing of "Seal Girl" Saoirse, and the magical adventure begins... This film is adapted from Irish folklore.