It was winter. Sirius had succeeded in tracking down most of his stuff, but he was still missing his prized stamp collection, and most of his silverware had vanished. Harry Ron and Hermione were coming to stay at Grimmauld Place for the winter break.
"Hey, Sirius, where did you go when you went behind the curtain? You never said.", asked Hermione.
"Well, I went to this land. It was called Bottom Earth. Their world was in a separate continuum than ours. There were a lot of similarities, though."
"Tell us about it!"
"Well, see, there were these little creatures called Hippits. And there was this evil dude, who put his soul into a necklace so that you couldn't kill him without destroying the necklace first. And then this one Hippit named Photo went on a quest to take it to a mountain to destroy it. It was a very, very long quest and it took three 500-page books with tiny print to tell it."
"Why didn't they just take the subway?" That was Ron. They gave him a look. Sirius cleared his throat and continued.
"Anyhow, I made friends with the evil guy while I was there. Not that I wanted to, or anything, but I figured that if anyone knew the way out of Bottom Earth, it was him. And I was right! He did know the way!"
At this point, the three children were silently confirming their thoughts that Sirius was totally bonkers.
"So, I told him about my world and about Voldemort. And he said that he knew the guy. Gave him the idea to put his soul into objects and everything. Said that he even had one of the objects with him. It was a bracelet. He gave it to me."
"How did you get home?"
"I eventually found a hole in the fabric of the space-time continuum. I went through it, and, well, here I am!"
"But.. then that means that the bracelet is still in Bottom Earth! Remember, the Hippit took it from you in chapter 2."
"Hurray! That means we'll have to go to another world to get it! This book is going to surpass the lengths of all of the other ones!"
The line about taking a subway comes from another conversation I had with a friend about LotR. I just had to use it.