Out in interstellar space, in another galaxy, a mysterious portal brings people to a planet with ruins from an alien race, the Undying, which had sent a broadcast telling of their own death, and promising tests, and rewards for those who succeed.
Jules and Mia are among those there. Mia scavenging for alien tech, for money she needs desperately. Jules for knowledge that he needs equally desperately.
When a chance encounter with each other and two more criminal scavengers leads them to a brief alliance, Jules persuades her to come to a location where he promises better tech. This leads to a quest through a ruin, involving their both lying to each other, harmony, serious questions about the Undying, nautilus spirals, the reasons for their desperation, the dangers of the dig, mercenaries, the government, stairs, mathematical and linguistic problems, and more.
It's a duology, and more like a novel cut in two than two closely related novels.