Looking for Winston

Sophie Mouse's little brother goes missing in the fourth book of The Adventures of Sophie Mouse.

Sophie Mouse has exciting plans for the day-she and her friends are going to build a top-secret fort at Butterfly Brook! As she sets off, she realizes that her brother, Winston, has tagged along. He's too little to build the fort with them, so Sophie sends him home. Then Sophie realizes that they could use an extra hand, and she turns back to invite him. But Winston is nowhere to be found. Where did he go? Will Sophie find her little brother?