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.

In an alternate 1941 France, where technology is dominated by coal and steam, April, her talking cat, and a young man named Julius go in search of her scientist parents who disappeared on the brink of a major discovery, as scientists have done for decades.