It started out with soil samples. They were strange soil samples, though. If poured out of their little glass or plastic tubes, they came out almost oily. They'd been suppressed, for some reason, the kind of evidence nobody was supposed to talk about. But the person who had 'em (in the dream, the proprietor of my local coffee shop, though I think he was supposed to be a soil scientist) knew the terrible story behind them, one of those stories for which the world is not yet prepared. But he had a small audience, and told the story.
Basically, the Joker was trying to steal a semi truck full of some plot device or other. While driving through the countryside, he heard weird noises from the trailer and pulled over. When he opened it, he found he had the wrong truck. He'd stolen a truck full of smuggled illegal immigrants.
You would think that the story would turn deeply nasty here -- except that the little girl who was the first to peer out of the trailer was holding a clown doll. For some reason, this charmed the Joker, and he decided to keep her on as a sidekick. He locked them up in the trailer again, fired some air holes along the tops of the walls with a Thompson gun, and took off to get a few things he'd need, for some reason or other.
And of course, he got knocked out by Batman and hauled off to Arkham Asylum.
The Joker managed to escape a few days later and made his way back to the truck. It was where he left it. And, of course, the people inside had had no food or water, so all of them were dead.
The Joker blamed the farmers on whose land he'd left the trailer, so he killed them and used their farm equipment to till the bodies of the dead immigrants into their fields.
The full details of the case, known as the "Field of Blood" case in the GCPD, were kept from the public by the police because knowledge of it would undermine confidence in the police and Batman.
I have had some screwy dreams in my life, but this one was remarkably coherent and really scary. Gahhhh. Can't sleep; clowns'll eat me.