A group of players start by discovering that one of their friends has been bitten by a vampire. They follow through the entire process, possibly killing their friend once he/she has risen again, probably hunting down the vampire that bit their friend. Happy ending.
Then the vampire community seeks retribution. Yes, it was clumsy of the vampire to get caught, but it's not the place of the herd to exact justice on the vampires. The complexity of this scenario depends upon how you imagine the entire supernatural community.
One possible idea is that vampires -- the cool manipulative Undead -- just don't exist. Vampires are mindless creatures which reek of clotted blood and which fixate on their families because those are the strongest memories left. A vampire is what happens to someone who dies of a ghoul- bite. (Doesn't happen often because ghouls don't usually bite live people. NOTE: these are obviously not _Vampire: The Masquerade_ ghouls.) The image of the vampire is the result of a plot between the ghouls and the werewolves: they wanted a patently false supernatural image that would distract attention from themselves. In this case, the PCs are under attack because they have a sample vampire to look at and modern science may discover the connection.
If you're running _Vampire: The Masquerade_, then the PCs are initiated by a Sire for their own protection. The Sire has some long- standing grievance against the Sire of the clumsy vampire, or has some ideological conflict with those who would kill the PCs.