A is a vampire who has lived in a cave for centuries.

B is his great great great great greatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreatgreat grand son who, after a lot of digging, discovered him and brought him back to the modern world.

This of course means that B has to watch his Grampa like a hawk to make sure he doesn't stick a fork in an outlet. or try drink bleach. or talk to strangers online.