Peg's mom has given her and Cat five important letters to mail. "She says it's because we're growing up," says Peg. And sure enough, the height marks on the wall show that they're getting taller! But there's a problem: when they get to the mailbox, they find a very big dog sitting in front of it. How can they get around or behind this scary-looking canine to mail the letters? Or could it be that they're jumping to conclusions -- and the obstacle itself...