Mamma Mia!

This article's plot summary may be too long or excessively detailed. Please help improve it by removing unnecessary details and making it more concise. (February 2015) (Learn how and when to remove this template message)
On a Greek island called Kalokairi, bride-to-be Sophie Sheridan posts three wedding invitations ("I Have a Dream") to different men.
Sophie's bridesmaids, Ali and Lisa, arrive. Sophie reveals that she found her mother's diary and has three possible fathers: Irish-American architect Sam Carmichael, Swedish adventurer and writer Bill Anderson, and British banker Harry Bright. As Sophie wants her father to give her away at the altar, she invited them without telling her mother, believing that after she spends time with them she will know who her father is ("Honey, Honey"). Villa owner Donna Sheridan, Sophie's mom, is ecstatic to reunite with her former Dynamos bandmates, wisecracking author Rosie Mulligan and wealthy multiple divorcÃ©e Tanya Chesham-Leigh, and reveals her bafflement at her daughter's desire to get married. Donna shows off the villa to Rosie and Tanya ("Money, Money, Money"). The three men arrive; Sophie smuggles them to their room and explains that she sent the invitations. She begs them to hide so Donna will have a surprise at the wedding: seeing the old friends of whom she "so often" favourably speaks. They overhear Donna working (humming "Fernando") and swear not to reveal her secret.
Donna spies them and is dumbfounded to find herself facing former lovers ("Mamma Mia"), and is adamant that they leave. She confides in Tanya and Rosie ("Chiquitita") that she is uncertain which of the men is Sophie's father. Tanya and Rosie rally her spirits by getting Donna to dance with the female staff and islanders ("Dancing Queen"). Sophie finds the men aboard Bill's yacht, and they sail around Kalokairi ("Our Last Summer") and tell stories of Donna as a carefree girl. Sophie plans to tell her fiancÃ© Sky about her ploy, but loses her nerve. Sky and Sophie sing to each other ("Lay All Your Love on Me"), but Sky is snatched for his bachelor party.
At Sophie's bachelorette party, Donna, Tanya and Rosie perform as Donna and The Dynamos ("Super Trouper"). The festivities are interrupted by the arrival of Sam, Bill and Harry. Sophie decides to talk with each of her prospective dads alone. While her girlfriends dance with the men ("Gimme! Gimme! Gimme! (A Man After Midnight)"), Sophie learns from Bill that Donna received the money to invest in her villa from his great aunt Sofia. Sophie guesses she must be Sofia's namesake and Bill is her father. She asks him to give her away and keep their secret from Donna until the wedding. Sophie's happiness is short-lived as Sam and Harry each tell her they must be her dad and will give her away ("Voulez-Vous"). Sophie cannot tell them the truth and, overwhelmed by the consequences of her actions, faints.
In the morning, Rosie and Tanya assure Donna they will take care of the men. Bill and Harry are about to confide in each other, but are interrupted by Rosie. Donna confronts Sophie, believing Sophie wants the wedding stopped. Sophie says that all she wants is to avoid her mother's mistakes. Donna is accosted by Sam, concerned about Sophie getting married so young. Donna confronts him and they realize they still have feelings for each other ("SOS"). Tanya and young Pepper continue their flirtations from the previous night ("Does Your Mother Know"). Sophie confesses to Sky and asks for his help. He reacts angrily to Sophie's deception and she turns to her mother for support. As Donna helps her daughter dress for the wedding, their rift is healed and Donna reminisces about Sophie's childhood and how quickly she has grown ("Slipping Through My Fingers"). Donna admits that her own mother disowned her when she learned that she was pregnant. Sophie asks Donna to give her away. As the bridal party walks to the chapel, Sam intercepts Donna. She reveals the pain she felt over losing him ("The Winner Takes It All").
Sophie and Donna walk down the aisle as the band plays ("Knowing Me, Knowing You"). Donna tells Sophie that her father could be any of the three men. Sam reveals that while he left to get married, he did not go through with it, and returned to find Donna with another man. Harry confesses that Donna was the first and last woman he loved and he has begun a relationship with a waiter from the taverna. The men agree that they would be happy to be one-third of a father for Sophie. She tells Sky they should postpone their wedding and travel the world as they have wanted. Sam proposes to Donna ("I Do, I Do, I Do, I Do, I Do"). She accepts and they are married. At the reception, Sam sings to Donna ("When All Is Said and Done"), which prompts Rosie to make a play for Bill ("Take a Chance on Me"). All the couples present proclaim their love ("Mamma Mia" reprise). Sophie and Sky sail away ("I Have a Dream" reprise).
During the principal credits, Donna, Tanya and Rosie reprise "Dancing Queen", followed by "Waterloo" with the rest of the cast. Amanda Seyfried sings "Thank You for the Music" over the end credits, followed by an instrumental of "Does Your Mother Know".

Question: Why did Donna's mother disown her?
Answer: Donna was pregnant