Prospero created a wicked tempest to shipwreck his brother Antonio's ship. Also aboard the ship is the King of Naples and his son, Ferdinand. No one on the ship dies in the tempest, but they are separated from one another--this is all part of Prospero's plan. Ferdinand is left to wander the island alone. Ariel, Prospero's personal spirit, tells Ferdinand that the King has died in the tempest.
Prospero and Miranda are watching from the side. Miranda has never seen another man before, so, at first, she thinks Ferdinand is a spirit of some kind. Ferdinand sees Miranda and introduces himself as the King of Naples, considering he believes his father to be dead, and offers to make Miranda queen.
Prospero sees that Miranda and Ferdinand have exchanged loving glances and becomes protective of his daughter. Prospero thinks Ferdinand is weak, and knows that he and Miranda have complete control over Ferdinand. Prospero also does not want to give himself away, so he accuses Ferdinand of being a spy and threatens to kill him.