A young girl named Nim lives on a remote island with her marine biologist father, Jack. They share the island with various animal friends and lead a content, adventurous life. Nim's favorite pastime is reading books, particularly those featuring Alex Rover, a fictional hero created by an agoraphobic author named Alexandra. When Jack sets out on a scientific expedition and becomes lost at sea, Nim finds herself alone and in trouble. Desperate, she reaches out to the only person she believes can help—Alex Rover.

Back in San Francisco, Alexandra receives Nim's plea for help and, despite her debilitating fears, decides to venture out to assist her. As Alexandra makes her perilous journey to the island, she battles her own insecurities and phobias. Meanwhile, Nim must summon all her courage and resourcefulness to protect her home and itself. The story unfolds in a series of thrilling and heartwarming moments, exploring themes of bravery, friendship, and the strength of the human spirit.