Sixteen year-old Juno MacGuff is the type of girl that beats to her own drummer, and doesn't really care what others may think of her. She learns that she's pregnant from a one-time sexual encounter with her best friend, Paulie Bleeker. Juno and Paulie like each other, but don't consider themselves to be exclusive boyfriend/girlfriend let alone be ready to be a family complete with child. Although she would rather not be pregnant, Juno is fairly pragmatic about her situation. Although there, Paulie really leaves all the decisions about the baby to Juno. Initially she decides that she will have an abortion, but that's something that she ultimately cannot go through with. So she decides to have the baby and give it up for adoption. But first she has to tell her father, Mac, and stepmother, Bren, that she is pregnant. Although they would have preferred if Juno was on hard drugs or expelled from school, Mac and Bren too are pragmatic about Juno's situation.
The next step is to find prospective parents for the yet unborn child. In the Pennysaver ad section, Juno finds Mark and Vanessa Loring, a yuppie couple living in the suburbs. Juno likes the Lorings, and in some respects has found who looks to be a kindred spirit in Mark, with whom she shares a love of grunge music and horror films. Vanessa is a little more uptight and is the one in the relationship seemingly most eager to have a baby. On her own choosing, Juno enters into a closed rather than open adoption contract with the Lorings - meaning she will have no contact with the baby after she gives it up. During the second and third trimesters of Juno's pregnancy which she treats with care but detachment, Juno's relationships with her family, with Paulie, and with the Lorings develop, the latter whose on the surface perfect life masks some hidden problems.