"And Professor Lupin stepped over you, and walked toward the dementor, and pulled out his wand, and he said, 'None of us is hiding Sirius Black under our cloaks. Go.' But the dementor didn't move, so Lupin muttered something, and a silvery thing shot out of his wand at it, and it turned around and sort of glided away.... "
-- Hermione Granger (PA5)
During a heavy rainstorm, a group of Dementors stop the Hogwarts Express as it travels to Scotland and search the train. One Dementor enters the compartment where Harry, Ron, Hermione, Neville, Ginny, and Lupin are traveling. Harry collapses as the Dementor approaches, but Lupin drives the Dementor away with a Patronus Charm (PA5).