Prospero, a powerful magician, uses his command over the spirit Ariel to cause the titular tempest that shipwrecks the mariners on the shores of the island on which the play is set.

The play opens with the sailors fighting the storm in act I, scene i. It is not until scene ii that the audience meets Prospero and Ariel and the tempest is revealed to have been supernatural in origin through the following exchange:

PROSPERO
Hast thou, spirit,
Perform'd to point the tempest that I bade thee? ARIEL To every article.
I boarded the king's ship; now on the beak,
Now in the waist, the deck, in every cabin,
I flamed amazement: sometime I'ld divide,
And burn in many places; on the topmast,
The yards and bowsprit, would I flame distinctly,
Then meet and join. Jove's lightnings, the precursors
O' the dreadful thunder-claps, more momentary
And sight-outrunning were not; the fire and cracks
Of sulphurous roaring the most mighty Neptune
Seem to besiege and make his bold waves tremble,
Yea, his dread trident shake.

Prospero's goal in sending Ariel to cause the storm was always to draw the sailors to the island, never to actually drown them or do them any physical harm. So after wreaking the havoc that Ariel describes in detail above, the spirit then ensured that the shipwrecked men made it safely to the shores of Prospero's island so that he could begin to exact his revenge.