Harry blew up Aunt Marge at Privet Drive and had to escape from the Dursleys in a hurry. Fudge, the Minister of Magic, kept him safe at The Leaky Cauldron until it was time to travel to Hogwarts School but, while on the Hogwarts Express, Harry fainted when a Dementor came aboard to search for the escaped convict, Sirius Black. The Ministry of Magic is worried that Sirius Black will try to harm Harry, so they are taking special care to keep him safe.