Marco Polo is a retired Christmas ornament, but in his mind he is a great mouse explorer. Because he's tiny and likely to be lost, he is the least-borrowed of the Book Buddies-toys that children can check out of the library just like a book-and he almost never leaves the library. But the little mouse finally meets the right match in Seth, a boy who is about to attend his first sleepover. If Seth were to bring the stuffed bunny he usually sleeps with, he'd risk being teased, but Marco Polo is the perfect size to hide away in his sleeping bag. Are both boy and mouse in for an unexpected adventure?