Plot

Ben, a young Irish boy, and his little sister Saoirse, a girl who can turn into a seal, go on an adventure to free the fairies and save the spirit world.

Saoirse is a child who is the last of the selkies, women in Irish and Scottish legends who transform from seals into people. She escapes from her grandmother's home to journey to the sea and free fairy creatures trapped in the modern world.—Anonymous

Ben and his younger sister Saoirse live with their father Conor in an isolated lighthouse. Their mother died six years ago while giving birth to Saoirse. Unable to talk, Saoirse is starting to show special powers and sea creatures seem to respect her, indicating that she is no normal little girl.—grantss

This Oscar-nominated animated feature tells the story of the last Seal Child's journey home. After their mother's disappearance, Ben and Saoirse are sent to live with Granny in the city. When they resolve to return to their home by the sea, their journey becomes a race against time as they are drawn into a world Ben knows only from his mother's folktales.

In 1981, Conor (Brendan Gleeson), a lighthouse keeper, lives on an island off the north-west coast of Ireland with son Ben (David Rawle), his pregnant wife Bronagh (Lisa Hannigan), and sheepdog Cu. Bronagh was very connected to Ben and encouraged him to sing, but he simply did not have the voice or the notes.

Bronagh tells Ben the story of how a Selkie sings a song to send all the Fairies home across the sea and then she converts into a seal. Bronagh gives Ben a seashell horn and says that it was given to her by her mother a long time ago. Bronagh says that in the shell, Ben can hear the song of the sea.

Bronagh disappears late one night, presumably dying after childbirth, leaving behind daughter Saoirse (Lucy O'Connell).

Six years later, Conor is broken, Saoirse is mute, and Ben is antagonistic towards Saoirse, blaming her for Bronagh's disappearance. Saoirse is often drawn to the seals in the sea who come to visit her at the beaches. On her birthday, she is visited by Granny (Fionnula Flanagan), the grandmother who regards the lighthouse as an unfit place to raise the children. Saoirse enters Ben's room and finds the seashell horn and is admonished by Ben. For Conor, Saoirse's birthday is also the day that Bronagh left the home. Try as he does, he cannot bring himself to be happy, making Saoirse sad.

That night, Ben scares Saoirse with a story of Mac Lir (Brendan Gleeson) and his mother Macha (Fionnula Flanagan), the Owl Witch, who stole his feelings and turned him to stone. Ben says that Macha has not stopped since. All the Fairies are afraid to come out on Halloween night, as if they do, Macha's owls will find them and take their feelings to turn them into stone.

Later, Saoirse plays a seashell horn given to Ben by their mother, leading her to a white sealskin coat in Conor's closet. She wears the coat and walks to a group of seals in the sea, revealing her as a Selkie (a person who can shape shift between human and seal). After swimming, she is found by Granny, who insists upon taking the children to the city. Conor reluctantly agrees despite Ben's protests, and locks the coat in a chest, throwing it into the sea. The chest falls to the bottom of the sea, along with its key.

Granny takes the children to her mainland home in Dublin, with Cu staying at the lighthouse. Ben did not want to leave, but Granny forced him to. Along the way, Ben draws up a map of the route, and on the eve of Samhain he and Saoirse run away from Granny's home to return for Cu.

Saoirse again plays Ben's seashell and is abducted by a trio of Daoine Si, taking her to their lair underneath a bushy roundabout. The Daoine Si rejoice at Saoirse's appearance, hoping that she can return them to their home world if she sings her song while wearing her coat. However, their jubilation draws the attention of owls belonging to a witch named Macha, who had turned the other Daoine Si to stone by sucking their feelings out of them. The owls attack the shelter and turn the Daoine Si to stone, but Ben and Saoirse escape. The children make their way toward the lighthouse to retrieve Saoirse's coat, and are reunited with Cu, who swam across the strait in pursuit of them.

During a rainfall, Ben and an ailing Saoirse take shelter in a hut, in which Saoirse falls into a holy well. Ci and Ben pursue Saoirse, but Ben gets lost and emerges in a cave inhabited by the Great Seanachai (Jon Kenny).

Through strands of the sage's beard hair, Ben sees the story of the giant Mac Lir, who cried out in a terrible fit of grief and threatened to flood the world. Macha, Mac Lir's mother, was unable to endure her son's anguish and thus deprived him of his feelings, turning him to stone.

Finding Saoirse's whereabouts through another strand, the Great Seanachai tells Ben that Macha has kidnapped Saoirse and that if Saoirse does not wear her coat and sing her song by dawn, she and all other magical beings will die. Following one of the Great Seanachai's hairs to Macha's lair, Ben sees a flashback of the night Saoirse was born and learns the truth: Bronagh was a Selkie, and she had to return to the sea so Saoirse could be born safely, leaving her husband and children behind.

Ben, in his encounter with Macha, learns that the grief-stricken witch seeks to save others from suffering and sadness by turning them to stone just as she had to Mac Lir. Macha herself has partially been turned to stone as she encloses her own strong feelings within enchanted glass jars. Ben finds Saoirse, who has nearly been turned to stone, in the attic. Apologizing for being a bad brother, Ben encourages Saoirse to play the seashell, which breaks the jars and returns Macha's feelings to her, restoring her kindness. Macha helps Ben, Saoirse and Cu return to the lighthouse, riding the wind, with the aid of two spectral hounds of the air.

As Saoirse is exhausted and near death, Conor attempts to take her to a hospital on the mainland, unwilling to heed Ben's explanation. Ben resolutely dives into the sea alone to retrieve the coat and unlocks the chest with the aid of a group of seals. Saoirse puts on the suit and begins to sing, which frees all magical beings from their petrified state.

As the beings depart for their home-world, Bronagh appears before the family to take Saoirse with her. When Ben and Connor plead her not to take Saoirse, Bronagh gives her daughter the choice to stay, as she is half-human. Choosing to stay, Saoirse removes her seal coat, thus untangling the two worlds forever. Bronagh shares one last goodbye with her family and departs.

Some time later, the no-longer broken family celebrates Ben's birthday, and he and Saoirse go swimming with the seals.