Pneumaticus

The newly-freed Sirius Black had recovered enough to make a good home for Harry. But he couldn't help but notice how thin and skittish Harry was. Harry never complained, and never explained about his abusive upbringing; but Sirius had the sense of it, having been an abused kid himself.

One summer evening, Sirius decided to get to the bottom of things. He dared Harry into a funny drinking game, so he'd be more relaxed, tongue-loosened and forgetful; then laced Harry's fourth drink with Veritaserum. A woozy Harry had to lie down on the couch, and Sirius extracted the whole sad, sordid story from him. Once he was done, Sirius told some jokes to clear his mind of the past. He let the boy just pass out with a big grin on his face, and handed him a good hangover potion first thing in the morning, and Harry couldn't remember a thing.

But Sirius remembered everything, and for once he didn't do the stupid impulsive Gryffindorish thing, but instead kept quiet, bided his time, and let out his Black and Marauder sides to plot revenge on the Dursleys for their horrible treatment of Harry. Of course, air-tight alibis would be needed.

After Harry had been back at Hogwarts for two months, and during a well-publicized trip Sirius took to visit the family sugar estates in Brazil, Sirius sneaked back into Britain for just one evening, (October 31,) using secret untraceable Black family portkeys to cover his tracks. Hidden under the disillusionment charm, he caught up with Vernon and Petunia, in their car at a service station, filling up the car with petrol and the tires with air, and had an inspired idea: he delivered two old Black family curses, laced with Black-designed memory charms that would make them think they'd always been this way. Sirius then portkeyed back to Brazil, nobody the wiser.

With the Pneumaticus! curse, Vernon was turned into a part-balloon! Any time he passed gas, or started shouting (which was often), Vernon would begin to deflate – and would have to make a trip to the service station to use the air-hose to reinflate himself through a valve in his belly-button. Eventually he had the good sense to buy a home electric air pump, and another portable one for the car. But there wasn't much he could do at the office; and he was forced to avoid spicy foods at lunch, and curb his temper around subordinates.

With the Automobilis! curse, Petunia was turned into a part-car! Any time she left the house or went for a walk, she developed an insatiable craving for the tastes of gasoline/petrol and motor oil, and the gas she passed smelled very much like car exhaust. Whenever she cleared her throat, she made little "Vroom vroom!" high-pitched noises. And of course, her urine burned and smelled of petrol – but Petunia was far too embarrassed and proud to see a doctor about it.

While Vernon and Petunia remained convinced that they were "perfectly normal, thank you very much," their neighbors and acquaintances became more distant. The Dursleys became virtual homebodies, having convinced themselves that they could do without the unfriendly and suspicious company of their former friends and snooty neighbors. And when Dudley came home from Smeltings for Christmas break, he was too dim and self-centered to notice any difference – aside from the occasional need to crack open a window.