The story begins on the Greek island of Kalokairi, where Sophie Sheridan (Amanda Seyfried) is preparing for her wedding to Sky (Dominic Cooper). Sophie has always been raised by her single mother, Donna (Meryl Streep), and has never met her father. She secretly reads her mother's diary and discovers that she has three possible fathers: Sam Carmichael (Pierce Brosnan), Bill Anderson (Stellan Skarsgård), and Harry Bright (Colin Firth).
Determined to find out who her father is, Sophie invites all three men to her wedding without telling Donna. When they arrive, Donna is shocked and upset, but she agrees to let them stay for the wedding. As the weekend goes on, Sophie gets to know each of the men and begins to wonder who her real father is.
Meanwhile, Donna is dealing with her own feelings about the past. She is still in love with Sam, but she is also afraid of being hurt again. She confides in her two best friends, Rosie (Julie Walters) and Tanya (Christine Baranski).
On the eve of the wedding, Sophie has a heart-to-heart talk with each of the three men. She learns more about their pasts and their relationships with Donna. She also learns that each of them has a special bond with her.
Ultimately, Sophie decides that she doesn't need to know who her father is. She is happy with the love and support of her mother and her friends. She marries Sky, and they all live happily ever after.