(From The Oxford Companion to English Literature:)
Changeling, The, a tragedy by Thomas Middleton and William Rowley, printed 1653, but acted as early as 1622.
Beatrice Joanna, daughter of the governor of Alicant, is ordered by her father to marry Alonzo de Piracquo. She falls in love with Alsemero and in order to avoid the marriage employs the ill-favoured villain De Flores, whom she detests but who cherishes a passion for her, to murder Alonzo. To the horror of Beatrice, De Flores exacts the reward he had lusted for. Beatrice is now to marry Alsemero. To escape detection she arranges that her maid Diaphanta shall take her place on the wedding night; and to remove a dangerous witness, De Flores then kills the maid. The guilt of Beatrice and De Flores is revealed to Alemero, and they are both brought before the governor, whereupon they take their own lives. The title of the play is taken from the sub-plot, in which Antonio disguises himself as a crazy changeling in order to get access to Isabella, wife of the keeper of a madhouse. The main plot is taken from John Reynolds's God's Revenge against Murther (1621).