Harry Potter had lived a very long life; he had mastered magic as very few ever had; but his deepest regret was that he had lost so many loved ones when he was so young, and had missed them for all of his long life. For years, he had been brewing a plan: to send his spirit back to his very first year at Hogwarts, and prevent all the deaths that had followed. After long research, he was ready.
Using elaborate runes and ritual circles within circles, Harry sat in the center and chanted. First, the runes began to glow. Then Harry began to glow. Then all went white.
Harry came back to himself, merging with his younger self, in the Gryffindor common room, sitting with Hermione and Ron. It was the night that Quirrell had gone after the Stone, and Harry and his friends would have to chase through the maze of traps to confront him. Dinner had just ended; the last time, they had nervously waited for all their fellow Gryffindors to go to bed – but Harry wanted to get down there early, and didn't want his friends endangered this time – nor would he need their help.
Harry excused himself to go up to his dorm and use the bathroom. Which he did; but on the way back down, he got his invisibility cloak and wrapped it around himself. On his way out of the portrait hole, he sent a wandless sleeping spell to Ron and Hermione, so they wouldn't fret.
Quietly, he made his way to the 3rd floor corridor; through the door, past Fluffy, down the trapdoor, and then made it through all the traps with ease, having the experience and power of a long magical lifetime to draw on. (Quite sensibly, he held onto the broom from the first room, so as to simply fly over the enchanted chess set.) When he arrived at the final room – where he spotted the Mirror of Erised – all was quiet. He got busy drawing runes on the back of the mirror, and then drawing another ritual circle around the whole area surrounding the mirror, and charming the circle invisible. Then he settled back in the corner of the room, invisibly, to wait.
After about 90 minutes, Quirrell showed up. He was nattering and whining to Voldemort, but kept wincing in pain. Spotting the mirror, he stood before it and looked into it. He walked around the back, muttering to himself.
Quirrell came back out from behind the mirror and stared hungrily into it.
"I see the Stone... I'm presenting it to my master... but where is it?"
It was just as Quirrell put his hands onto the mirror that Harry pointed his wand and whispered, "Activate!" Before Quirrell could react, a swirl of colored lights swept around him and the mirror.
Quirrell disappeared; in his place, lying on the ground, was the Stone.
Looking into the mirror, Harry saw Quirrell – now trapped inside it – ceremoniously handing the Stone to Voldemort. Both wore wide grins. Then the mirror went blank – and the scene repeated itself. Over and over. Quirrell and Voldemort were now endlessly reliving their fondest heart's desire; but were blissfully unaware that they were trapped in a pocket universe, nor realizing that there was a real world on the other side of the mirror.
With a smile, Harry cast a permanent unbreakable charm on the mirror, picked up the Stone and pocketed it, and made his way back to the common room, removing his cloak of invisibility and stowing it in a pocket. Sitting back down with his friends, he gently woke them up. "Had a nice nap?" he grinned, and pulled out the Stone to surreptitiously show it to his friends.
Hermione gasped: "How did you get that?"
Harry gave her his patented lopsided grin, and only said: "Magic!"
AN: Italics are from Chapter 17 of Harry Potter and the Sorcerer's Stone.
