A certain sixteen-year-old boy was sitting in the back of a car on the London Underground, about halfway to his destination, when the train stopped and all the lights went out.
Everyone else looked around in confusion, except for the boy, who was filled with a feeling of dread. This situation was way too familiar… Then the cold set in.
“This is not happening again,” he muttered. Because of the events of the previous summer, he’d bought a second wand, untraceable to him, and had been practicing nonverbal spells with it, especially the Patronus Charm, Stunner and binding spells. He had quickly mastered those, then moved on to slightly more dangerous spells.
He surreptitiously removed his backup wand from its leg holster, aimed out the back of the car, and fired off a message Patronus to Hermione, Remus and Tonks, and Ron and Ginny. He didn’t want the other passengers to see it and be able to recall later the distinctive figure of a stag. He then Stunned everyone else in the car so he wouldn’t have to Obliviate them later.
Once they were all unconscious, he released Prongs toward the front of the car, sending him after the dementors who had just begun to approach the car, the last one on the train. Harry didn’t even want to think about what had happened to the passengers of the other cars. He had to concentrate on his happiest memories—his friends Ron and Hermione, his godfather Sirius, Remus and Tonks, the Weasley clan—and his love for them all in order to keep the Patronus working.
“Tell whoever sent you this time that I’m tired of their games! If it’s that hag Umbridge, tell her she won’t get away with it a third time, that I’ll come directly for her next time, and if she’s lucky I won’t bring a couple of you with me! Now get out of here!” Harry sent Prongs after them again, and Prongs charged into the middle of the group, scattering them.