The novel opens with simple description of the landscape of Costaguana (which we can only assume translates to "The Coast of The Iguanas," right?), the fictional nation in which events take place. There are no characters in this chapter, and we get no firm idea of when in time this description comes from. By opening the novel this way, Conrad may be trying to give us a hint that this isn't the most plot-driven story in the universe. Thanks for the heads up, Conrad.
It takes a couple of chapters for a clear set of plot points to emerge, and even then, there's a ton of jumping around. That being said, we'll lay out the novel's events for you in more or less linear form, because we're total sweethearts like that.
The story proper really begins with Charles Gould's decision to take over his father's silver mine in Costaguana. An Englishman who was born in this fictional territory, Gould decides that he's going to make the mine productive (which his father was never able to do) and use the mine to bring stability and peace to his war-happy birthplace. So, he hauls his bride-to-be back to the coastal town of Sulaco and gets to work. What could possibly go wrong here, huh?
When the mine becomes a success, the Goulds grow ridiculously powerful and influential in Costaguana politics—yay. However, Charles's hopes regarding encouraging peace and prosperity don't really work out—boo. Basically, immediately after the Goulds help maneuver Don Vincente Ribiera into the position of President/Dictator, rebel forces (led by General Montero, who had previously served as Ribiera's Minister of War) start agitating to overthrow him, and war breaks out.
You might be wondering where our title character has been in all this. We were wondering, too. Not to worry, though—Nostromo definitely plays an important role in the novel's events. An Italian sailor who became the foreman of the Oceanic Steam and Navigation (O.S.N. for short) group of lightermen and caretaker of the Sulaco jetty, Nostromo is basically everyone's go-to guy.
He helps the town get rid of thieves and protects the Violas (i.e., the family he lives with), and he's instrumental in helping to get Don Vincente out of Costaguana before the rebel forces can seize him. His name sounds a tad occult, but he's a good dude with a sound head on his shoulders.
During the war that ensues, Nostromo is asked to help the Goulds get one of their silver shipments out of Sulaco before rebel forces arrive in town, so they can't get their grubby rebel paws on it. As part of the same mission, he is supposed to get Martin Decoud out of town. Martin, a journalist who is really critical of Montero and his people, would have been in mortal danger once Montero got to town. The plan is to meet up with a passing boat and put Martin and the silver on there.
They manage to get out of Sulaco successfully, but not without some hiccups. While trying to sail quietly out of the harbor, they discover a hysterical stowaway named Hirsch on board. Then, an incoming steamer filled with rebels, led by Sotillo, hits their lighter, causing it to flood. In the brouhaha, Hirsch goes overboard and gets picked up by the steamer.
Meanwhile, Martin and Nostromo end up at the Great Isabel, an island off Sulaco's coast, where Nostromo drops Martin off and they bury the treasure, pirate-style. He leaves Martin with a lifeboat, but Martin is basically supposed to stay put until Nostromo can come back.
Nostromo heads back out in the lighter. To make sure his mission and its outcome are top-secret, he sinks the lighter and then swims back to Sulaco to hide out and figure out a new plan. He ultimately helps out the resistance efforts by traveling to Cayta to bring General Barrios back to Sulaco. He ends up being super-instrumental in the creation of an independent Occidental Republic.
Martin is not so lucky: he ends up going crazy on the island and shooting himself. Nostromo decides to keep the silver they had buried there for himself, since it was already assumed lost when the lighter sank.
However, that decision ends up being his downfall. Greed is not good, Nostromo. The O.S.N. decides to build a lighthouse on the Great Isabel, which makes Nostromo pretty anxious that someone will find the silver. To solve that problem, he make sure that his friends the Violas get installed there as caretakers. He gets engaged to the eldest Viola daughter, figuring it would give him the perfect excuse to be on the island skulking around.
However, while he's sneaking around one night visiting his treasure, old Viola mistakes him for another dude who was trying to date his youngest daughter, Giselle (irony alert: Nostromo actually had been sneaking around making kissy face with the younger Viola… he just isn't doing it at that moment) and shoots him. Nostromo then dies.
Moral of this story: burying treasure, pirate-booty-style, almost always goes wrong.