Even though twelve-year-old Alexis was born in Hawaii, she won't surf or swim with her friends—not since the ocean and its hidden creatures swept her out to sea. Instead, she grabs her best detective hat and decodes her mom's latest challenge.

Alex's mom works in counterintelligence and leaves codes, ciphers, and puzzles behind for Alex to solve, always with a "treasure" at the end. It's a brilliant game between them, and Alex loves figuring out her mom's puzzles—especially the tricky ones—but when an emergency at sea puts her mom in possible danger, solving the next one suddenly feels far more urgent.

Friends help as Alex races to decipher each clue before time runs out, but when the trail leads to grumpy old Uncle, his enormous dog Sarge, and a sea turtle unlike any other, the challenge changes into something bigger than any before. With storms on the horizon and lives on the line, Alex must face her fears to solve Mom's challenge and save those she loves. With her ohana to help, she must be strong like the sea.