Despite warnings from her teacher and headmistress, Jenny (Carey Mulligan) goes to Paris with David (Peter Sarsgaard) to celebrate her 17th birthday. They sight-see, have a picnic overlooking the Seine, and Jenny loses her virginity to David. She doesn't understand how sex could inspire the passion it does in books and music.
After they come home, he proposes and she accepts. She leaves school without taking her final exams. On the way to a celebratory dinner with her parents, Jenny finds mail addressed to "Mr. and Mrs." in David's glove compartment. She confronts him but he refuses to explain. She goes to his house, finding David's wife and young son, and learns he was unfaithful before and got a girl pregnant.
Jenny's headmistress refuses to allow her to repeat her senior year. Jenny studies on her own and with the help of her teacher, takes the final exams, and is accepted to Oxford. Jenny is shown enjoying her classes, spending time with fellow students, and looking forward to a trip to Paris with her new boyfriend... which she pretends she's never seen.