"I'm going. I've had enough."
-- Harry Potter (PA2)
Marge insults Harry’s father, and Harry’s deep anger leads to uncontrolled magic. Marge inflates like a balloon and floats up to the ceiling. Knowing he’s in trouble with the Dursleys for blowing up Marge and believing he’s expelled from Hogwarts for performing magic outside of school, Harry grabs his school trunk and runs away from Number Four Privet Drive (PA2).
Outside in the dark, Harry is startled by large black creature watching him. He inadvertently signals for the Knight Bus, which picks him up in Magnolia Crescent. As the bus careens around Britain, Harry falsely gives his name as Neville Longbottom and the conductor, Stan Shunpike, tells Harry about Sirius Black and his recent escape from Azkaban.
When the bus skids to a stop outside the Leaky Cauldron, Cornelius Fudge (and Hedwig) meets Harry on the sidewalk and tell him he’s not in trouble (PA3):
“Two members of the Accidental Magic Reversal Department were dispatched to Privet Drive a few hours ago. Miss Dursley has been punctured and her memory has been modified. She has no recollection of the incident at all. So that’s that, and no harm done.”
— Cornelius Fudge (PA3)
Since Marge stayed for a week, arriving on 31 July, it is logical that she would be leaving after seven days, on the morning of the eighth day. In that case, this dinner party happened on 6 August. Harry gets to the Leaky Cauldron early the next morning.
This event provides Rowling with a prime opportunity to talk about one of her favorite themes: that prejudice and intolerance and self-centeredness are very, very bad things indeed. Aunt Marge is all of these things rolled together into one large, mustachioed package, complete with an evil-tempered animal alter-ego with a violent name whom she treats far better than the humans around her. This chapter is great fun to read and Marge's comeuppance is delicious and perfectly suited to her: a woman whose sense of self-importance is inflated far beyond reason is herself inflated and ends up bobbing about on the ceiling, turned harmless and helpless and utterly laughable. In the next chapter we learn that the remedy for this is "puncturing." How wonderful. Too bad we don't get to hang around the Dursleys to see this done.
Earlier, Marge says, "I won't have this namby-pamby, wishy-washy nonsense about not hitting people who deserve it." It seems she gets her comeuppance. After blowing Marge up, Harry tells Uncle Vernon, "she deserved it." -BB
When insulting James and Lily on the last night of her visit, Marge is drunk. Ironic, then, that she accuses Harry's parents of not only being worthless layabouts but also of being drunk when they died in that car crash.
The big black dog, of course, is really Sirius Black in his animagus form. This makes Uncle Vernon's remark upon learning of Sirius on the Muggle news a week earlier, "Lunatic could be coming up the street right now!" into a reality.
Aunt Marge tells Harry that if he had been dumped on her doorstep that she would have sent him straight to an orphanage. Harry was bursting to tell her that he would rather live in an orphanage than with the Dursleys. If the Dursleys had sent Harry to an orphanage, he would have ironically grown up the same way as Tom Riddle. Harry knows this from his encounter with Riddle the year before. The information in the note that Dumbledore left with baby Harry on the Dursleys' doorstep prevented this from happening. If Harry had grown up in an orphanage, he would have had yet another similarity with Voldemort. Dumbledore had to have known this and would have rather had Harry endure eleven years of bad treatment from the Dursleys than to share something else in common with Voldemort.
Aunt Marge would make a very good Voldemort supporter if she were a witch and not a Muggle. She, like the Malfoys, have a real problem with not being a pureblood. She makes this clear when speaking about dog breeding. "Bad blood will out," she tells Petunia. Aunt Marge considers one of the basic rules of good breeding is: "If there's something wrong with the bitch, there'll be something wrong with the pup-"
When Harry finally snaps after enduring a week of Aunt Marge's insults toward him and his parents, his emotions get the best of him. Harry having never felt so angry in his life never goes for his wand or utters any curses; the magic just seem to emanate from him. Harry just "blew up" because he could not take Aunt Marge's nasty comments any more, and appropriately enough Aunt Marge blew up, too. Harry knows too many curses from his previous two years at Hogwarts, so why did his emotions choose to inflate Aunt Marge? Could it be that this happened because he spent the summer watching Dudley continue to balloon in size?
Marge eats, drinks, and does many things to excess. There seems to be a double entendre in Harry saying, "I'm leaving. I've had enough." -BB
This flight down darkened streets parallels his adventure in the same neighborhood at the beginning of the fifth book. Then the streets are familiar, not frightening, but the dangers are much more real. In that instance, he is being watched over by the Order of the Phoenix. Now he is being watched over by Sirius in the form of a huge black dog. In book five, the most immediate danger comes from the Minister of Magic, the very person who appears at the end of this chapter to rescue Harry from his predicament.
A very detailed and accurate map of the streets around Privet Drive is available here in the Lexicon. The artist, Nik, actually visited a neighborhood which matches the descriptions in the book and walked the streets as it describes in this chapter and in the first chapter of OP. Of course, since Little Whinging doesn't actually exist, no map can be considered canon. However, this is undoubtedly the best representation we can hope for. The street names of Little Whinging are somewhat similar to street names in Chipping Sodbury, where JKR grew up.
It was apparently that hand motion which summoned the Knight Bus, but this cannot be the whole story. Can you imagine what it would be like if the Knight Bus turned up whenever someone happened to fling out their wand arm? This is actually yet another example of the power of intention in magic. Not only did Harry happen to make the correct hand gesture (quite by coincidence) but also he was in the correct frame of mind to need emergency transport. That combination is what summons the Knight Bus. Another slight possibility is that someone besides Harry summoned the Bus. Sirius is a logical candidate, although he is in dog form and therefore probably couldn't make the required gesture--to say nothing of the fact that he likely doesn't have a wand. Another possibility is Mrs. Figg, but that seems unlikely because Mrs. Figg would have had to have been in exactly the right place at the right time and it also assumes that Squibs are capable of calling the Knight Bus.
The name "Knight Bus" is a play on words. You can actually catch a "night bus" in London, although they are much harder to come by than the magical version. There aren't a lot of them, but they do rumble around London all night. If you are ever stranded, seeing that bus come along is just about as welcome a sight as the Knight Bus was to Harry.
The Knight Bus can apparently be anywhere in Britain within seconds, even Anglesea, which is an island (so the bus can go over water, just not UNDERwater).
Madam March turns up again on the Knight Bus in OP24, poor woman. The phrase "slightly green" refers to the fact that she's rather badly off with motion sickness, not that her natural coloring is greenish. In the OP incident, she actually vomits all over the second deck of the bus. Madam Marsh is, according to Rowling, simply an "extra" with no further part to play in the saga (JKR).
Contrast this with Fudge's attitude about this very same incident two years from now, during Harry's disciplinary hearing on another charge (OP8).
Of all the names Harry could have chosen for the alias that he gives Stan Shunpike, he chooses Neville Longbottom. As we learn in GF and OP, Harry and Neville have many traits in common: both were born at the end of July, both had parents in the Order of the Phoenix, both sets of parents narrowly escaped Voldemort three times, and in the end Voldemort was directly or indirectly responsible for causing both Harry and Neville to be reared by people other than their parents. Harry does not learn any of these facts until the end of his fifth year at Hogwarts making this a very ironic choice for Harry to use Neville as his alias.
An example of the series' symmetrical structure at work: This event toward the beginning of the third book is very similar to Umbridge being carried off by the centaurs toward the end of the fifth book (OP33). In the two cases, similarly unpleasant women go too far in insulting someone, and and get the just deserts for all their cruelty - in a dramatic, almost deus ex machina -like way. -BB