I've read the original Rendezvous with Rama but I never got around to reading any of the sequels. So I never got the straight story on how the whole series ends. From what I gather from the Wikipedia article on Rama Revealed:
The Rama spacecraft rendezvous with an enormous "Tetrahedron" near the star Tau Ceti, at a place designed to research any intelligent life capable of spaceflight.
The Rama aliens are apparently collecting intelligent life from around the galaxy using their Rama spacecraft, but why are they doing this?
Why are the Rama aliens sending Rama spacecraft around to collect intelligent space-faring species?