When Jack Holden salvages an abandoned sailboat, the Sea-Lark, he sets up a ferry business. But strange events start happening that threaten his new enterprise. First, shadowy figures sneak aboard late at night, trying to steal something. Then Jack catches another prowler red-handed and a struggle ensues. After that, two shady men hire the Sea-Lark for a sail but turn on Jack and his friend Rodney, marooning them. Why are these men so desperate to get their hands on an old sailboat? Jack is determined to solve the mystery and save his precious Sea-Lark. With new dangers around every corner, he races against time to uncover long-buried secrets and bring the culprits to justice.