We pick up the story 20 years after Gateway, with that novel's protagonist, Robinette Broadhead, now a comfortable millionaire on an increasingly resource-starved future Earth. He now funds space expeditions to find more of the technology of the Heechee. The Heechee were an extremely advanced alien race that left technology lying around the solar system (and the galaxy) after they disappeared half-a-million years ago.
Humanity started exploiting that technology after the first Heechee artifacts were found in artificial tunnels on Venus about 50 years before this novel. But humanity doesn't know how much of the technology works, especially the faster-than-light stardrives on the remaining Heechee spacecraft.
These spacecraft, mostly housed at the carved-out asteroid dubbed Gateway, can be programmed with unknown destinations and sent out into the galaxy. Desperate 'prospectors' pay into the Gateway Corporation for a chance to ride on these ships. Sometimes they come back with new artifacts or useful locations; sometimes they come back with nothing; sometimes they don't come back, or come back dead.
Robinette's fortune was made by the disastrous but lucrative discovery of a non-rotating black hole, a discovery that dropped everyone but Robinette into said black hole. Now, he's sent an expedition to a newly discovered Heechee vessel in our Oort cloud. It seems to be a Heechee food factory, using carbon, hydrogen, oxygen, nitrogen, and trace elements found in the cometary halo around our solar system to make a nearly inexhaustible supply of food. It could solve Earth's food problems. But where is the factory periodically shipping its food?
Well, there's the question. The Gateway books are the much smarter father to such SF series as Stargate SG-1 and Babylon 5 in which humans seek out ancient alien technology, often using it without any idea how it works. Broadhead and the other sympathetic characters of this novel, including Wan, the most lost of all Lost Boys, and an ancient machine consciousness that was once a living, non-human (is it a Heechee?) being, are skilfully drawn.
There's less sense of the unknown in this second book, as Pohl begins to answer more questions than he asks. But the big questions, Where Did the Heechee Go and Why Did They Go There?, only begin to be answered here. The answer is a big one, but it awaits the sequel. Recommended.
The Gateway Saga 3: Heechee Rendezvous by Frederik Pohl (1984): The original Gateway Trilogy wraps up here as the mysterious Heechee finally put in an appearance 500,000 years after they seemingly vanished from the universe. The action of the third book picks up about 20 years after the second, with trilogy protagonist Robinette Broadhead beginning to show the effects of old age despite having a really terrific health plan.
Among Pohl's achievements here is a prediction of our current Cloud-based computer world, though in Pohl's world processing comes from a gigantic global pool upon which all computers can draw. That's some network! This future Earth's health care, though, hasn't been so predictively accurate, perhaps because Broadhead's health problems are such a key part of the narrative.
Many of the mysteries first posited in Gateway back in 1977 are solved here, most notably the reason for the Heechee's long disappearance. Unfortunately, the narrative bogs down again and again with Robinette Broadhead and his improbably beautiful, hyper-intelligent, and financially successful wife. She's part wish fulfillment and part comic relief, the latter because of her Boris-and-Natasha manner of speaking English. What larks, Pip!
Indeed, the trials of Broadhead and Wife seem to fascinate Pohl a lot more than the science fictional mysteries he himself created. The ending, when it comes, is rushed, somewhat perfunctory, and clearly left open-ended for yet another sequel. There would be three more Gateway novels, if you're counting.
The novel really is a must-read if one has read the first two Gateway books, despite its immense and multitudinous flaws. The tendency of the narrative to spin off into interpersonal whoopsy-cutesiness reminds me of similar problems in Robert Heinlein's later-career novels; Pohl composed the Gateway Trilogy between the ages of 57 and 64, along with a boatload of other novels.
The going never gets as bad as it does in Heinlein's worst moments (see: The Number of the Beast), but the going, she is rough at times. Pohl's decision to transform a character from the previous novel into an annoying sociopath doesn't much help things, though it does pose an interesting question as to why he did so. My guess would be that he had a conversation with a psychologist who suggested that the character's upbringing, as constructed by Pohl, would almost certainly create a near-monstrous sociopath. But it's too bad -- and the scenes with that character are almost unreadable.
Oh, and there are black holes, weird black holes, artificial black holes, and various space fleets and space whales and thingamajigs. Despite all the lavish praise heaped by the novel on Broadhead's wife, the novel also posits two alien species with major downsides for females: Heechee women go into heat and, if they don't have sex while in heat, have a pronounced tendency to die; a second race not only has non-sentient females, but non-sentient females who are a food source for the (sentient) males as well as breeding stock. Well, alrighty, then! Lightly recommended.