A team of scientists has gone to Titan, Saturn's largest moon, to study alien structures that were abandoned thousands of years ago. Nobody can figure out what the function of them is, except that they are machines, and that they are still operational after millenia. Dr. Sydney Lee has a gut feeling that the purpose of the machines is not beneficial to humanity. In fact, he believes that the makers, known only as "The Others" have left them running in order to harm Earth. He is afraid that the aliens could come back, that they are our enemies. Feeling isolated and paranoid because nobody will believe him, at the beginning of the book he is just barely stopped from committing suicide.
Fast forward to a year later. Instead of waiting for The Others to return, the authorities on Earth have decided to send out fact finding missions to habitable star systems. Out of 40,000 candidates, only 80 will be selected to go on the missions, some of which will take years,decades even, to fufill. After Lee checks out of a mental hospital, he is one of the few selected to go to Sirius. Going along with him is Marlene Ettinger, his former lover from the Titan mission. Although he had cut her out of his life, they are still in love, whether they want to be or not. Further complicating things is Dan O'Bannon, her current lover, who is helpless to do anything about her fixation on Lee. O'Bannon is being sent to Jupiter to look for signs of The Others.
As on a Darkling Plain had great characters. The love triangle was well done. They weren't the usual stupid sci-fi characters, which are usually the result of bad writing. The long journeys to find alien races reminded me of Haldeman's The Forever War, which actually came out about four years after this book. The only complaints I have is the implausibility of some of the alien life forms in the book. It also kinda puttered out at the end. Anti-climax. Well, it was a good book, withstanding those things.