As the four Callahan cousins spend winter break at their grandmother Gee's estate on Gull Island, they fill their time with such activities as sledding, snowman contests, caroling, and a drama-packed sleepover at the Gull Island Whaling Museum.
The twelve-year-old Callahan cousins are staying at their grandmother's rambling seaside estate for a whole summer without parents, but when the usually shy Kate tries to become the "cool" girl in town, her antics threaten to pull the cousins apart.
As their summer on Gull Island continues, the four twelve-year-old Callahan girls busy themselves with redecorating the dorm, dealing with their mean neighbor Sloan, and uncovering a secret about their family's past.
