Note: This is taken from an authentic dream I had recently. Don't ask me to explain, because I cannot.
The Batman of Earth 7 had the same kind of Batcave that all the other Batmen do: a hole in the ground filled with bats, stalagtites and standing water. There's the obligatory giant penny and various other trophies. But there are telling differences: a close examination of a newspaper photo of the Joker reveals that the Joker of Earth 7 is a black man wearing white face. There are pictures of Batman riding at the head of a horse cavalry, backed by a vanguard of white-hooded Klansmen. In a place of honor sits a photo of Batman shaking hands with President Woodrow Wilson.
The Batman and Robin of Earth 2 had traveled across the dimensional barrier to visit their Earth 7 counterparts. They had actually flown in a strange transdimensional airliner established by strange extraterrestrials for unknown reasons. At the airport, Robin had been approached by a mysterious man in the bathroom and asked if he could smuggle some contraband aboard the ship. The Boy Wonder readily agreed. The strange man pulled a clove of garlic from his satchel. It was the largest clove of garlic Robin had ever seen - easily as big as a man's fist. The man asked Robin if he knew what to do, and Robin said that he did. To the man's surprise, Robin was able to fit the entire clove up his anal cavity. Little did Robin know, however, that the garlic had been soaked in an extremely strong extraterrestrial LSD beforehand.
The Batmen strolled leisurely around the Batcave, while Robin was diverted in a private screening room. It was not considered smart that the Dick Grayson of Earth 2 know too much about the fate of his Earth 7 counterpart (who had become an FBI agent and was involved in hunting bankrobbersand bootleggers alongside J. Edgar Hoover), so he was sequestered in a small theater to watch comedy reels while the Batmen chatted. But once ensconced in the theater, Robin was unable to watch the silent films. Instead, in his mind he imagined that the entire room was totally black, and that instead of a movie projected on a screen, he saw a disembodied head slowly fellating a giant tubular clove of garlice. The man was wearing a wide-brimmed hat, however, and his features were obscured. But as he continued to fellate the garlic his face came into clearer focus. He had no eyes, and the space where his eyes should have been was black and hollow - filled only by a glowing, hollow crescent moon where his right eye should have been. Robin screamed and felt as if the bottom of the world had disappeared.
Meanwhile, the two Batmen reminisced about their past team-ups. The Earth 7 Batman was on a rant about the pernicious influence of "the damn kykes and niggers and Communists on the moral cohesion of the United States". Earth 2 Batman listened politely, not objecting but silently glad that this would be their last cross-world meeting. Soon, on another world, another Bruce Wayne was fated to lose his parents. Earth 2 Batman had discovered this world - a world which, for various complicated reasons, would be designated Earth 1 - and ensured that the child's parents would meet the same grisly end that his own had, and as had the parents of the Batman of Earth 7. But the Earth 1 Batman would be raised on a gentler world than the rather unpleasant Earth 7 from which the older Batman hailed. Once they left this planet for the last time, Earth 2 Batman would eradicate every trace of the older Batman's reality, ensuring that the bigot Batman, the harsh Batman who appeared in a cameo during Birth of a Nation and who kept a running tally of every lynched black man carved onto the running board of the Bat-Flivver, would soon be forgotten. And soon he would have adventures with the heroes of Earth 1, and no one would ever remember the inhabitants of Earth 7.
One day, he imagined, the Batman of Earth 1 would do the same to his world, Earth 2, similarly ensuring that another Bruce Wayne on another world would one day grow up to become the grim avenger of evil. And so on, until the end of time - the Batman was a self-perpetuating virus spreading throughout the history of the multiverse.