When Anna was little, she and her mother used to search for sea glass, but since they looked at night, they called it moonglass. Now, ten years after her mother's mysterious death, her father is working as head lifeguard on the same beach where her mother grew up and her parents first met and fell in love.
Reluctant to get close to anyone (including her father) and not pleased about having to start at a new school, Anna begins to spend more time alone, running the length of the beach and wondering about who her mother really was. After meeting a lifeguard named Tyler, she slowly lets her guard down and together they start exploring the abandoned houses that dot the beach.
But when learning more about her mother's past leads to a painful discovery, Anna must reconcile her desire for solitude with ultimately accepting the love of her family and friends.