So, here's the plan for the first "arc" that's floating around in my head.
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We introduce all of the characters while on the hunt for Jack The Ripper in London. He's just butchered a very young girl (14) and left her mutilated in the streets, and we all discover her. This then prompts the group to finally put an end to Jack once and for all. However, contrary to their secrecy they can't inform the public or government about his death as they are a secret order, so the entire population still believes him to be alive and well.

After we track him down and kill him, we will soon learn that the first main villain, a vampire known as "Olaf" is residing in London and planning on infecting the general water supply with his blood to turn the entire population of London into vampires.
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Sound good?