How about this: There are 100 immortals that live on Earth. They're not immortal in the traditional sense; instead the always reincarnate immediately after dying. Basically they die, ~9 months later they're born again. They slowly regain their memories of previous lives as they grow up.
Since they've lived hundreds, maybe thousands, of lives; they have built up power bases (corporations, cults, royal lines, etc.) that they reassert control over when they reach adulthood.
Your PCs could be employed by one of the immortals in their fight against other immortals or part of an organization fighting them.