Houseboat Mystery

The Alden children, Henry, Jessie, Violet, and Benny, are now living with their grandfather and spending their summer vacation on their aunt's houseboat. As they travel down the river, they encounter strange events such as a leaky boat, a missing figurehead, a replaced rudder, and a mysterious note in a bottle. The children soon realize they are being watched, and discover that someone is searching for something on their houseboat and is determined to find it.