Gateway is a hollow asteroid constructed by the Heechee, a long vanished alien race. The Heechee left behind nearly a thousand small starships. Many still work, but using them is an extremely dangerous gamble, since Heechee technology is mostly incomprehensible to humans. Attempts at reverse engineering to find out how they work have ended in disaster. The controls for selecting a destination have been identified, but nobody knows where a particular setting will take the ship or how long the trip will last. Once in flight, no one has changed the settings and ever been heard from again. Most settings are to useless or lethal places; however a few lead to Heechee artifacts and habitable planets, making the passengers (and the Gateway Corporation which administers the asteroid on behalf of several countries) wealthy.
The vessels come in three standard sizes which can hold a maximum of one, three, or five people, crammed in with equipment and (hopefully) enough food to last the trip. Each ship comes with a lander to visit a planet or other object if one is found.
Robinette (Bob) Broadhead is a food shale miner on Earth who has won a lottery, giving him just enough money to purchase a one-way ticket to Gateway. Once there, he loses his nerve, putting off going on a mission as long as he can, but eventually he starts running out of money. Although terrified, he goes out on three trips. The first draws a blank. On the second, he makes a discovery through unauthorized experimentation, but this is balanced by the fact that he has to pay for the ship he managed to destroy in the process.
On his third trip, the Gateway Corporation tries something different: sending two five-person ships, one slightly behind the other, to the same destination. Bob signs up, along with Gelle-Klara Moynlin, a woman he fell in love with on Gateway. When they reach the end of their journey, they find to their horror that they are in the gravitational grip of a black hole, without enough power to break free.
One of the others comes up with a desperate escape plan: to cram all the people into one ship and eject the other toward the black hole, thus gaining enough velocity to escape. Working frantically to transfer unnecessary equipment to make room, Bob finds himself stuck alone in the wrong ship when time runs out, so he sacrifices himself and closes the hatch. However, his ship is the one thrown away, leaving the rest of the crew falling into the black hole.
He returns to Gateway and becomes wealthy when, as the sole "survivor", he gets paid for the entire group. He feels enormous survivor guilt for deserting his crewmates, especially Klara, so he seeks therapy from an Artificial Intelligence Freudian therapist program which he names Sigfrid von Shrink. He finally comes to terms with his guilt despite the realization that, due to the gravitational time dilation resulting from proximity to the black hole, time is passing much more slowly for his former crew mates and none of them have actually died yet, leaving him with the dread that Gelle-Klara believes he betrayed them to save himself.
The novel is divided between chapters of dialogue between Bob and Sigfrid and chapters covering the main action. Also embedded are various mission reports (usually with fatalities), technical bulletins, and other documents Broadhead might have read.
(Source: Wikipedia)