There are lots of children on Hill Street, but no little girls Betsy's age. So when a new family moves into the house across the street, Betsy hopes they will have a little girl she can play with. Sure enough, they do—a little girl named Tacy. And from the moment they meet at Betsy's fifth birthday party, Betsy and Tacy becoms such good friends that everyone starts to think of them as one person—Betsy-Tacy.
Betsy and Tacy have lots of fun together. They make a playhouse from a piano box, have a sand store, and dress up and go calling. And one day, they come home to a wonderful surprise—a new friend named Tib.
希尔街上住了很多小朋友，但都比贝特西大。但对面的房子里新搬来了一家人，他们有一个比贝特西小的女孩！自从在贝特西的5岁生日上见面后，两人就形影不离。