Bruce Wayne was a drunk.
At his Halloween Party, he wanted to dress up as Batman.
His butler Alfred tried to convince him that this was a bad idea.
They wrestled for a bit, and Bruce had Alfred in a headlock as the butler smashed a vial of knock-out gas into Bruce’s face.
They both passed out, and the party went on without them.
Everyone had a great time, and nobody noticed that Bruce was missing.
But they noticed that Aflred was missing.
The caterers ransacked the manor.
Bruce woke up, got into his Batman suit, and hunted them down.