What has happened:
Once there was a girl named Katy. She had a wonderful life and wonderful parents. Nothing could ever happen wrong with her world.
Then her parents died in a car accident, leaving her alone on her 16th birthday.
For a while, she lived alone in her old house, watched over by her friend's parents, The Ichijojis. She was always sad, though. The only thing that brought her happyness was the nightly visits from her unnamed friend, the boy who popped into her dreams at night.
Finally, the boy left her dreams and became a reality. He was really the vampiric Digimon Myotismon, who had just been reborn. They fell in love quickly, especilly after Katy saved Myo from death at his evil half's hands. She had almost died herself in doing so, leaving her part digimon, and able to shift from Digital Monster to Human at will. There were a few nasty scars from it, and she did use up energy really quickly,but other than that she was suffering no ill effects.
But Myo always felt that he was in debt to his girlfriend, and was desperate to find a way to make it up to her. Even when she told him that just staying with her forever would be enough, he still felt that there was more that he could do.
A week later, they all went on a vacation to the Uinta mountains in Utah. Willis, an american Digidestined, invited them to go camping. They thought it would be fun and relaxing. But Apocalymon had other plans. He sent Piedmon to either enlist Myotismon's help, or kill him along with the Digidestined. Myotismon almost went with him, when Piedmon slipped up and mentioned killing Katy, who is really the only reason Myo puts up with the Digidestined. Myotismon came to his senses and refused Piedmon's offer.
Piedmon tried to kill Katy, but Myotismon jumped in the way, saving her life at the cost of his own. But he promised Katy he would come back one day.
Afterwords, the Digidestined were split up. The first half was led by Ken Ichijoji, and they still called themselves the Digidestined. The Digidestined now consisted of Ken, Davis, Yoli, Tai, Sora, Mimi, and Cody, and all of their Digimon.
But the other half was called the Midnight Guardians, and was led by Katy. The Midnight Guardians were forbidden to ever call themselves the Digidestined ever again, and had been banished from the Digidestined. They were the ones that trusted Myo, and eagerly awaited his return. They included: Katy, Matt, Tk, Kari, Izzy, Joe, Willis, all of their Digimon, and Lady Devimon(who had survived to battle against Piedmon).
2 years later, fog gathers on a cold december night, growing thicker and thicker like a gray curtain over Tokyo...