The story is mostly set a few years from now, about the discovery and raising of what appears to be an alien artifact from the sea floor. But it also follows the life on our planet of the creature that arrived in that artifact (Haldeman calls it "the changeling," who has been here so long it's forgotten its own origins), and another rather nasty non-human creature ("the chameleon") - and both of them can and do masquerade as regular humans. The story alternates between the slow socialization of the changeling starting around 1930, and the current day tale of the research into the artifact.
I suppose I hoped for more of a revelation around the nature of the changeling at the end of the story. There was something, but not even its reasons for being on our planet were explained. But the story was well constructed, the characters well drawn, and the writing was good, so it was an enjoyable read.