The crossword solutions for the term QUARTET IN 'MAMMA MIA!' are EMS, which stands for the four main characters in the musical 'Mamma Mia!'. These characters are Donna Sheridan, her daughter Sophie Sheridan, Sam Carmichael, and Bill Anderson. Donna is the owner of a Greek island hotel and Sophie is about to get married. Sam and Bill are two of Sophie's three possible fathers. The quartet of characters is central to the plot of the musical.