Here's what that sporadically truthy "encyclopedia" called Wikipedia has to say about The Birth of Merlin:
"The Birth of Merlin" possesses a three-level plot, a structure common in plays of its era. On the first level, the main plot, the characters are royal and their concerns are those of statecraft and national welfare; in the second-level plot, the characters are aristocratic and genteel and their concerns are those of personal values and personal fulfillment; and on the level of the comic subplot, the characters are common and their concerns are largely sensual.
Unusually, the play begins on its second level: the opening scene introduces the nobleman Donobert, his daughters Constantia and Modestia, and their suitors Cador and Edwin, and begins the story of Modestia's conflict between her desire for a religious vocation versus social pressures to marry. The famous characters of Arthurian romance do not appear until the second scene, which introduces King Aurelius and his royal court.
The first scene in Act II introduces the otherwise-unnamed Clown and his very pregnant sister, Joan Go-to't. References through the play identify the Clown a typical Rowleian fat clown, the type of role that Rowley repeatedly wrote for himself to play. The Clown's sister has gotten herself pregnant by yielding to the advances of a mysterious stranger; she and the Clown are now wandering through the forest, searching for the father of the child, or at least a father for the child.
In a cave in a forest, the Devil summons Lucina and the Fates to attend Joan as she gives birth to Merlin. The Clown catches up, to meet his sister and his new-born nephew, a fully-grown Merlin the Magician. Merlin introduces his Clown-uncle to his Devil-father; the Devil predicts a dramatic future for his newborn son.
The play is rich with visual effects of varying types, including devils and magic and masque-like spectacles. It was clearly designed to provide broad, colorful, fast-paced entertainment.
Further details about Catclaw's production of The Birth of Merlin, which is possibly the first full-scale production of the play in the last 200 years, will be announced soon.