The concept album was Summer's (and Disco's) first double album, telling a modern-day Cinderella-themed story through means of disco music. The album's story concept was conceived by Joyce Bogart, Susan Munao and Donna Summer based on an idea by Al Bogatz. The songs were written in collaboration between Summer, Giorgio Moroder and Pete Bellotte. The album was her last to be recorded at Musicland Studios in Munich. Arrangements were handled by Bob Esty while electronic arrangements were the work of Moroder. The artwork was designed by Stephen Lumel and Gribbitt! with photography by Francesco Scavullo.
Summer's previous two records were musical concept albums: 1976's Four Seasons of Love told the story of a love affair by relating it to the four seasons, while 1977's I Remember Yesterday presented a musical catalogue of musical styles and lyrical themes from the past, present and an imagined future. Once Upon a Time is another concept album—the first "disco opera" per Robert Christgau—developed by Joyce Bogart, Susan Munao and Donna Summer as a modern-day Cinderella narrative.
Throughout the album and as described in the liner notes storyline, the songs tell the story of a young woman who lives in a fantasy world of make-believe in which she is seemingly trapped, but thanks to her belief in her dreams she embarks on an adventure that ends with the man she loves entering her life. The "rags to riches" story is brought into the modern day via the use of the electronic disco sound.
The album sold reasonably well as it made the US Top 30 and was certified gold in the U.S. by the RIAA that same year. It spawned the European hit single "I Love You", which became her fifth Top 10 in the UK in less than two years, and it also hit the US Top 40. "Rumour Has It" was also a UK Top 20 hit.