Mark his [Antonio’s] condition and th’ event. Then tell me
If this might be a brother. (I.ii.)

Prospero says these words to Miranda in his account the story of his exodus from Milan. Prospero draws on the language of kinship to emphasize the gravity of Antonio’s betrayal. Although Antonio’s plotting with Alonso constitutes treason, the fact that Antonio was his brother made the transgression even more devastating. Antonio’s act of treason serves as a model on which Shakespeare builds other examples of treasonous behavior in the play.

Th’ occasion speaks thee, and
My strong imagination sees a crown
Dropping upon thy head. (II.i.)

Antonio utters these words to Sebastian when the other members of their retinue have fallen asleep due to Ariel’s charm. Although Antonio appears simply to be reminding Sebastian of his noble heritage, he is in fact implying that Sebastian should expedite his ascendance to the throne by killing his brother in his sleep, thereby becoming the King of Naples. Sebastian initially seems aghast at Antonio’s suggestion, but he quickly grows amenable to the idea and joins the treasonous plot.

Yea, yea, my lord. I’ll yield him [Prospero] thee asleep
Where thou mayst knock a nail into his head. (III.ii.)

Caliban speaks these words to Stephano, whom he has convinced to assassinate Prospero and take control of the island. Although Caliban’s desire to “knock a nail” into Prospero’s head echoes the other plots of treason in the play, it differs in the sense that Prospero and Caliban are not related. Thus, Caliban and Stephano’s plot does not constitute a familial betrayal. However, the basic motivation for the murder plot remains the same as the others, since Caliban maintains that Prospero has unjustly laid claim to the island over which he—Caliban—should rule.