Even with magic, gemino and such, McGonagall realizes that she has to keep replicating Harry's letters until he gets to read it because the Boy Who Lived is coming to Hogwarts, dammit. So the first time McGonagall sends another letter. Just one. The Dursleys confiscate that one, too. So she sends three in the hope that one of them might be overlooked. Mr. Dursley boards up the mail slot and Minerva just goes Game on, muggle. So she multiplies her duplicates tenfold. She communicates with the owls to do their worst, deliver the letters in any way possible, which the owls gleefully oblige, since Vernon has been doing his best to hit them with his car.

Filch doesn't understand why ten owls are missing one day, fifty the next, and by the time McGonagall is about to send the hundreds of letters flying out of the chimney at the Dursley's residence, the school is out of owls to use, so McGonagall calls in a favor with the post office in Hogsmeade, and when the post office runs out, she goes to Diagon Alley and even buys herself four more owls (to keep in touch with former students, she tells herself, but really she's this frustrated and willing to be petty so she buys four owls of her own even though she's a cat person).

But even with the hundreds of borrowed owls, McGonagall is both surprised and delighted to find that hundreds more arrive of their own accord. What she understands but can't exactly know is that the owls have been gossiping about the horrible muggle house with the raging beefy man who tries to hit them with his company car, his umbrella, his briefcase, anything to keep them away. All the owls seem to be eager to join this fight and have been leaving pellets on the doormat and soiling the windows of Vernon's car for weeks so he has to go through several car washes before he can see again.

Imagine Minerva McGonagall staying up all night, duplicating letters until she's whispering gemino in her sleep and she wakes up to find she has extra chairs and photographs and nightstands and she has to vanish them all, but there is the enormous stack of letters to Harry Potter at the foot of her bed and she is filled with enormous wicked delight at the thought of Vernon Dursley's face when the owls deliver no fewer than seven hundred and seventy-seven letters (probably more because she nodded off at that number, but kept casting spells). She adds something close to a summoning charm tied to Harry's location so the letters will be attracted to whichever room Harry is in when they arrive.

As a cat, she's watched the Dursleys in her spare time since she, Dumbledore, and Hagrid dropped off baby Harry on the doorstep. She's heard Vernon's verbal abuse of the boy who lived, she's seen him hit the child, and even though it distastes her to do so, she makes sure to pee on his shiny car whenever he gets a new one. She's seen Petunia give preferential treatment to Dudley, but she has also seen Petunia be kind and gentle to baby Harry when he's extra fussy. Petunia may be distasteful to Minerva, but Petunia is Lily Evans's sister and there is some bond between Petunia and her nephew. It's Vernon that lights a fire under Minerva that makes her rage, but she doesn't dare use magic near Harry until he's at Hogwarts. She's been waiting for this kind of opportunity for eleven years.

So it is with enormous satisfaction that Minerva ties each letter individually to each owl's leg. Each owl gives her a polite nod of solidarity, and she makes sure to give extra owl treats to the veterans who have made every flight to Number Four, Privet Drive and who have been leading this small revolt. Minerva makes a trip to Hogsmeade to apparate a block away from Privet Drive just so she can watch. (She greets Mrs. Figg's cats when she visits.) The owls do not disappoint. From across the street in a neighbor's hedge, Minerva can see Vernon chortle at the idea that mailmen do not deliver post on Sundays, and she can see the first letter fly past Vernon's nose, taking half his mustache with it. She can see the owls practically throwing letters (as best owls can do) down the chimney and she sees them come rocketing out the other end (thanks to her extra enchantments), all seven hundred and seventy-seven and more shooting into the Dursley's sitting room and there is Harry, snatching and grabbing at letters that go flying past him.

At that moment, Dumbledore sends a small speaking patronus, a tiny phoenix roughly the size of Minerva's paw, and calls her back to Hogwarts. Minerva casts one last look at the rumpus that has become the Dursley's house, now just a flurry of letters through the window, and apparates back to Hogsmeade. Dumbledore is waiting for her in his office (Password: Ice Mice) and before she can say anything, Dumbledore just starts giggling. This is the most Minerva has ever seen Dumbledore laugh, even for the good humored fellow that he is. And before Minerva opens her mouth a second time to speak, Dumbledore just says through tears of mirth: "he still didn't open one" and continues laughing. Minerva's enraged reaction (which is to pick up and throw a couple delicate instruments since she knows he doesn't actually care about trinkets) only makes Dumbledore laugh harder.

They agree to send Hagrid because he doesn't need magic to make Vernon Dursley cower and squirm. McGonagall is both pleased and furious that Hagrid succeeded on the first try.