How does Caliban's evil plot get thwarted by Prospero and Ariel

Caliban, Stephano, and Trinculo come looking for Prospero, and swipe a few garments of Prospero's on their way. Caliban still wants very much to kill Prospero, and carry out this plot; however, Trinculo and Stephano are very drunk, as usual, and prove completely incapable of anything but petty theft. Prospero catches them‹not difficult, since they are making a huge amount of noise--and sends Ariel after them as they flee.