It concerns a secret society of people genetically predisposed to longevity. The members of the society gain a special grant if they marry one of the members with the right heredity. The idea being to extend their lifespans.
The general populace discovers their existence, and a "pogrom" ensues. This crisis precipitates an exodus into the stars. After many adventures, the exiles are forced to return home.
Because of time-dilation effects, centuries have passed on earth. To their surprise they find that on earth longevity is commonplace. Spurred on by a belief that there was some specific "technique" to the Howards longevity, they have explored every avenue known to science to duplicate the feat. And have succeeded through surgery and advanced molecular chemistry.