The more traditionally science fiction part of the story deals with the first starship from Earth to arrive in another star system - Alpha Centauri. A number of starships have been sent out to look for habitable planets as new homes for an overpopulated Earth. The Alpha Centauri stars and their planets are older than our solar system. Exploration of the system finds evidence of alien civilizations which began billions of years ago and disappeared long ago. The planets have aged through all this time and don't offer a promising new home for humans. A device which gets data from the quantum level gives them extrapolated views of what those lost civilizations were like and how they disappeared.
That story is mixed with and skewed by issues among the crew (originally 10 men and women, but one didn't survive suspended animation on the trip there.)
Some readers may be uncomfortable with the amount of description of various sexual activity - including some non-consentual or nonstandard. In many cases, this is associated with something about a particular character. The captain is a woman who grew up in a cult community in which adolescent girls were ritually used in orgies. The engineer was born male (still using "he"), but later had a medical procedure making his obvious genitalia female, yet being able to bring out male. The planetologist has multiple personalities and is associated with a secret society "Indigo" - in which men are infected with a sexually transmitable organism(?) which leaves women sterile. Once out of suspended animation, he starts to seduce / infect the female crew members. I'm not sure that this required as much sexual detail as the book contains, but it's not merely about "mating season."
That being said, there does seem to be a lot of various hooking up of different combinations of crew members. This may partly be to suggest why Earth has become overpopulated. It's also because we're shown several crew member's psychological damage from their younger days. Perhaps, this is meant to suggest that the kinds of people who would volunteer to leave almost everything from their past life behind and travel light-years away to a previously unexplored star system may have some serious issues which make such a choice easier. One way or the other, these aspects play a big role in the book. Readers should consider where such a book belongs in their priorities.
In the end, the device tells the crew that the "spider" aliens stagnated and withered away because they lacked real goals and drives to give them reason to continue. Perhaps, the author is trying to tell us that the kinds of things that can emotionally damage us and twist us also keeps us going. It's not really the kind of question I seek to explore.