Ten-year-old Julia and her family are spending their summer on the Shetland island of Unst, where her father has been hired to automate an old lighthouse. Julia's mother is a marine biologist and she hopes to spend the time finding and studying the elusive Greenland shark, a species that can live up to 400 years, but that has only been seen a few times. As her mother returns empty-handed each day and her grant applications rejected, Julia sees her fall into despair and begins to worry. When her mother's behavior takes a turn for the worse, Julia takes drastic action to find the shark on her own and places herself in grave danger.