Why did Shakespeare include the sub-plot of Antonio and Sebastian scheming against King Alonso?
In Act II, scene i., as King Alonso and his trusted advisor Gonzalo lay sleeping under Ariel's spell, Prospero's brother, Antonio, suggests to the king's brother, Sebastian, that they kill Alonso so that Sebastian can become King of Naples. The plot is Antonio's brainchild and he broaches the scheme to Sebastian by saying "My strong imagination sees a crown/Dropping upon they head" (II, i., ll.208-209). Not only is Sebastian slow to get Antonio's drift, at the last minute he stays their swords from the heads of the sleeping King and the worthy counselor, and Ariel re-appears to awaken them. At this point, the conspirators jettison their scheme. The question becomes: Why did Shakespeare insert this minor intrigue into the play given that it goes nowhere so quickly? The sub-plot appears to perform a number of relatively minor functions. It highlights the diversity of the activities that take place on the Island, some of which are magical and extraordinary others of which and mundanely human. Second, it allows us to distinguish between a passively evil Alonso, a character who undergoes a full transformation and is admitted to the play's concluding masque, and the actively evil Antonio, a character who acknowledges his guilt but is not fully redeemed. Lastly, it suggests that "imagination" is not always a powerful force for good, it can also be a force for evil and an object of ridicule.