Complex sentence: Critics called "The Understudy" a "return to form", and, while it was based upon "Macbeth", a knowledge of the play was not necessary for enjoyment; the episode's divergence from the play was praised.
Split into simpler sentences: Critics called "The Understudy" a "return to form". While it was based upon "Macbeth", a knowledge of the play was not necessary for enjoyment, and the plot's divergence from the play was praised.