Although both the complete libretto (1690) and the full score (1691) of Henry Purcell's first semi-opera "The Prophetess" or, "The History of Dioclesian" have been preserved, the work has tended not to be fully discussed. In this study its relationship to the play of the same name by John Fletcher and Philip Massinger (1622) and in particular the "Alterations and Additions After the Manner of an OPERA" by Thomas Betterton, actor-manager of the Dorsert Garden Theatre, come under scrutiny. A line-by-line comparison is made of the opera with the only two published versions of the play extant, which are in the Beaumont and Fletcher First Folio (1647) and Second Folio (1679). Double numbering is used to aid easy reference to both notes and folios. The alterations are shown to be classifiable under such headings as late 17th-century theatre conventions, political expediency, providing scope for music and dance, and unification. The author uses the high degree of unification found in the text and particularly in the music, which is dealt with separately, to show that earlier criticism of this semi-opera as lacking coherence is unfounded.