The stunning conclusion of The Daedalus Mission series.
The final contact made by the Daedalus Mission begins badly, even before the ship makes a hard landing in the middle of nowhere. The situation of the colony doesn't seem to make any sense, and neither does the situation of the indigenous aliens--the Sets--that have helped the colony survive and thrive. Alex Alexander doesn't take long to work out a hypothesis that might explain the mystery--a hypothesis that the people on the ground have already worked out for themselves--and he's fortunate enough to fall in with a colonist who's obsessively determined to prove the hypothesis. Unfortunately, the quest seems likely to become so dangerous that both of them might die trying--and there's too much at stake not to take it to the very limit of possibility, no matter what the cost.