Katherine, Mark, Jane, and Martha are in for a long, dull summer. They find a magical, coin-like talisman that grants the bearer wishes. The catch is that it grants half of any wish made by its bearer—a wish to be on a desert island sends them to the Sahara desert, and their mother ends up halfway home when she wishes to return home during a dull visit to her relatives. That "half magic" is a challenge, sometimes comical, until the children learn to double their wishes.