Mary Jane Parker was all set to spend Halloween at a costume party with her husband, the first time they'd been able to do that in a long time. Maybe even ever. But Peter suddenly had to work. He'd heard that the Green Goblin was back in town, and wanted to stop his arch nemesis before he could put any sort of plan into action. He promised Mary Jane he'd be there, probably late. So putting on her vampire costume, MJ goes without him.
Once she gets there, she finds that there is no party. She also finds that Peter should've come with her after all. A vampire and a spider, that could be an excellent combination. But mostly, it's because she found the host of the party: Norman Osborn.
I wanted to use a variation of the costume she wore in the Halloween episode of the Spectacular Spider-man series, only without the skirt. I had trouble with the collar, so I took it out too.