Passion is a musical written by Stephen Sondheim, based on a 1981 Italian film, Passione d'Amore which itself is based on a 19th Century novel Fosca by Iginio Ugo Tarchetti.

From the Life Like Company's website:

A remote military outpost in 19th century Italy, a handsome army captain, separated from his beautiful – but married – mistress, is forced to re–evaluate his beliefs about love when he becomes the object of the obsessive, unrelenting passions of his Colonel’s ordinary, ailing cousin.

Passion is one Sondheim's least performed works and often provokes strong reactions from audience members on the motivations and actions of the characters (refer to the Wikipedia article).