Quick Answer: Mamma Mia! is based on a lesser-known 1968 Italian film, Buona Sera, Mrs. Campbell.

In 2009, a stage musical premiered at London’s west end Prince Edward Theatre featuring the songs of iconic 1970s-80s Swedish pop group ABBA. The show, Mamma Mia!, tells the story of Sophie, a 20 year-old fiancée who wants her father to walk her down the aisle at her wedding. Unfortunately, she doesn’t know who her father is. After discovering her mother’s journal, Sophie uncovers three names, each of whom has the potential to be her father. Unbeknownst to her mother, Sophie invites these three men to the wedding in an attempt to discover her parentage. Comic and dramatic times ensue.

The show was adapted into a 2008 film starring Meryl Streep, Pierce Brosnan, Amanda Seyfreid, Colin Firth and an ensemble of other recognizable faces.

The three potential dads of Mamma Mia!, with the daughter

If this plot sounds at all familiar to anyone who hasn't seen Mamma Mia!, that’s because it isn’t a new story. Mamma Mia!’s plot line is based on the 1968 Italian-shot Hollywood sex comedy Buona Sera, Mrs. Campbell starring Gina Lollobrigida. In the film, Mrs. Campbell is an affluent and beloved woman living in a small Italian community. Her daughter, Gina (Janet Margolin), is away at school in Switzerland. Mrs. Campbell maintains that she is the widow of an American Army soldier who, shortly after she conceived Gina, was killed in action leaving her to raise the child alone. In reality, as a virginal 16 year-old during a period of ten days in World War II, she temporarily resided with three soldiers, and any of the three could be Gina's father. Like the teenage Mrs. Campbell, the men were all alone and “in need of comfort.” The result was Gina, as well as three monthly separate child support checks making their way to Italy for the past two decades. When a 20-year reunion brings the men back to the Italian village on the same weekend Gina is home from school, Mrs. Campbell frenetically works to maintain the cover story which has governed her life and to keep everyone from finding out the truth. Comic and dramatic times ensue.

Mrs. Campbell with Gina's three potential dads in Buona Sera, Mrs. Campbell (1968)

The core story of Buona Sera, Mrs. Campbell and Mamma Mia! are the same: a single mother trying to hide three potential fathers from the daughter she’s raised alone, honestly unaware of which man is the real parent. Both stories are fueled by strong maternal performances that offer both sentimentality and humor, a beautiful daughter who viewers can sympathize with, and three characteristically unique father figures whose unique personality traits offer identifiable differences.

Movies turning into stage plays and back into movies has since become a trend, with the likes of The Producers (1968, 2005) and Hairspray (1988, 2007) following suit around the time of Mamma Mia!’s success.