The Jungle starts with a wedding: gentle giant Jurgis Rudkus has finally married his young, fragile fiancée Ona Lukoszaite. The two are dancing at their wedding feast in Chicago's meatpacking district, Packingtown. While there are already hints that things are not well with this couple – Ona has to go to work the next day or else "It will ruin us!" (1.41) – Jurgis promises his wife that he will work hard and look after her.
Rewind a year or so to small town Lithuania. Jurgis falls in love with newly-orphaned Ona. He wants to start a family and get ahead. Jonas, Ona's step-uncle, has a friend working in Chicago. This friend has apparently struck rich in the United States. So Jurgis and his future in-laws have a chat, and they all decide to head to the States to find a better life. They are tired of the old country and its hierarchies and social rules. So off they go: Jurgis, Antanas (Jurgis's old father), Ona (the fiancée), Teta Elzbieta (Ona's stepmother), Jonas (Ona's step-uncle), Marija (Ona's cousin), and Ona's six stepbrothers and sisters (in order of age from oldest to youngest: Stanislovas, Kotrina, Vilimas, Nikolajus, Juozapas, and Kristoforas).
Once they arrive in the U.S., they discover that the streets aren't exactly paved with gold. They have decided to look for work in Chicago's busy industrial stockyards, where thousands of cattle are slaughtered and processed into canned and packaged meats everyday. Jurgis easily finds work because he is big and strong, but his father, Antanas, is elderly and can't handle hard labor. Not only that, but the living conditions for their family are pretty terrible, so they have to start looking for a house – which is way beyond their current means.
Things seem to be looking up when Marija (Ona's cousin) gets a job painting the outsides of cans of smoked beef and Jonas (Ona's step-uncle) gets a meatpacking plant position. Antanas also finally finds a job pickling meats. So, with four out of the twelve members of the family all working, they decide to put down a down payment on a house. The house has four rooms and there are twelve of them, but Teta Elzbieta sleeps in the same room with all of her children, and it somehow works out. The cost of living in America is a lot higher than they are used to, so their wages don't go as far as they had hoped, but the family is not (yet) truly desperate.
That all starts to change, though, when a neighbor explains that their house down payment doesn't guarantee them anything. If they miss a single monthly payment on the remainder of the money they owe, they will immediately be evicted. They also have additional, unexpected interest payments to come up with every month. So Ona (who is only fifteen) and Stanislovas (Teta Elzbieta's oldest child, a boy of thirteen) have to get jobs, too. With Jurgis, Marija, Jonas, Antanas, Stanislovas, and Ona all working, they should be fine. Believing that they will be able to survive as a family, Jurgis and Ona finally get married.
Then, winter comes – and winter in Chicago is no joke. Jurgis's grandfather gets a persistent lung problem that eventually kills him. It's also a slow time around the meatpacking plants, so Marija's factory temporarily closes down. Ona is just about to have a baby, so they are scrimping and saving for a doctor, but they are already down two salaries. How are they going to make ends meet? In these bad times, Ona gives birth to a healthy baby boy, whom they name Antanas in honor of Jurgis's father. Even though the birth has affected Ona's health, she still goes right back on the job because the family can't afford to lose any money.
Summer comes, and work picks up. Marija has gotten a job trimming beef at a factory. Things seem to be somewhat optimistic – until, of course, Jurgis is injured on the job. This is long before the days of worker's compensation, so he is laid up for two months to heal without any kind of salary coming in. And Jurgis has been the family's biggest earner! Jonas can't stand the poverty and hard work any more and disappears one day. The family has no choice but to send two more of Teta Elzbieta's sons, eleven-year-old Vilimas and ten-year-old Nikolajus, out to work. Teta Elzbieta, who had been looking after the family at home, also goes out to get a job at a sausage-making factory. So now, while Jurgis is in bed, the employed members of the family are: Teta Elzbieta, Ona (still around sixteen or seventeen), Stanislovas (thirteen), Vilimas (eleven), and Nikolajus (ten). And thirteen-year-old Kotrina (Teta Elzbieta's oldest daughter) has to stay home and cook and clean for the whole family while caring for baby Antanas and her youngest brother Juozapas. (Teta Elzbieta's youngest child, Kristoforas, has died of a sudden illness in the meantime.)
Slowly, even Jurgis (who is not the most perceptive man ever) notices that Ona has been changing lately. She is pregnant again, but she seems very unhappy. She has sudden fits of hysterical crying. Once or twice, she doesn't come home at night. She tells Jurgis that this is because the weather is so cold it's not safe to walk home, and that she has been staying with a friend of hers from work. Eventually, Jurgis discovers that Ona has been lying to him. What has actually been happening is that Ona has been raped by her boss, who has also blackmailed her into going to a brothel downtown to become a prostitute several times. Jurgis is so outraged to hear this that he runs out immediately and attacks the boss, Connor.
Jurgis is pulled off Connor and thrown in jail for aggravated assault. That's it, as far as the family's concerned: they are ruined. Their biggest earner, Jurgis, is out of commission for a whole month. Ona has become too sick to go into work. Stanislovas was unable to go to work for several days owing to extreme bad weather, so he lost his factory job and has had to join his younger brothers selling newspapers in the city. Teta Elzbieta's sausage factory has shut down for the winter, so she is begging in the streets. Kotrina has had to go into the city to sell newspapers with Stanislovas, Vilimas, and Nikolajus.
When Jurgis finally gets out of jail, he finds that his family has been evicted from their house. They are staying in the tenement building they used when they first arrived in the United States. Ona is incredibly sick – her baby is premature, and she has never been strong since Antanas was born. Jurgis begs Ona to stay with him, but she and the baby both die (Ona is, at this point, eighteen). Jurgis gets drunk.
When Jurgis comes home to Teta Elzbieta and his remaining family, he promises to sober up and stand by the family so that his son, Antanas, will grow up strong. Jurgis manages to find another job thanks to his old union connections from his previous factory days, and things start to look up again. But of course, Jurgis's luck is terrible: his new factory shuts down because there is too little demand for the farm tools they make. So Jurgis now has to beg and rely on the few pennies the children bring in from their paper selling in the city. He finds another job by chance at a new steel factory, Teta Elzbieta gets a job scrubbing office floors, and Marija finds a place as a beef-trimmer in the stockyards. Could it be that things are getting better?!
Nope. Jurgis comes back to the tenement house one day after work to find all the women of the household crying. With no one to watch him, Antanas (the baby, now about one and a half) ran out into the street and drowned in a puddle. So now Jurgis has lost both his wife and his son. Jurgis walks right out of Chicago and becomes a hobo for the summer. He picks up odd jobs as a migrant farm laborer, but he blows all of his cash on drinking and prostitutes. He has given up thinking about his future or family or anything that used to matter to him. This country has chewed Jurgis up and spit him out again, and he is incredibly bitter. If society keeps screwing him over, he is going to get revenge by destroying society.
So, Jurgis becomes a criminal. He falls in with a buddy who he met back in prison earlier on in the novel, Jack Duane. Jack Duane and Jurgis start mugging people in Chicago. Jurgis also becomes part of an election-rigging operation for a local rich man and swindler, Mike Scully. Scully gets Jurgis a job at a meatpacking plant so that Jurgis can secretly drum up votes for a candidate for city council among his worker friends. After the election ends, Jurgis stays on at the factory. Even though the union calls a strike to protest unfair labor practices, Jurgis keeps working – he has become what is known as a "scab," a worker who doesn't pay attention to union calls for a strike. Jurgis has stopped caring about anyone or anything except himself.
Jurgis has made a pretty comfortable (if dishonest) life for himself. By buddying up to Mike Scully, he has earned three hundred dollars to put in the bank. But it all comes crashing down when Jurgis happens to see his old enemy Connor. Jurgis attacks Connor a second time. He discovers that Connor is high up in Mike Scully's organization, so there is no way Jurgis will be able to dodge a long prison sentence. The only thing he can do is post bail (which winds up being $300) and get out of Packingtown. Jurgis flees the stockyards with three dollars in his pocket. Once again, he has to start begging on the streets to keep from starving.
What steps in to give Jurgis's life meaning again is socialism. Jurgis goes to a free political meeting to find somewhere warm to sit out of the Chicago cold. What he finds is a speaker who tells him that he is not alone in his troubles. All of the workers of the world are being oppressed by fat cat businessmen who control the political process so completely that there is no chance of change. The only thing workers can do is to organize among themselves to elect socialist candidates to government.
Jurgis is absolutely convinced by these arguments and becomes a truly faithful socialist. Jurgis finds a job at a socialist business, a hotel run by a prominent lobbyist for workers rights. And he starts hanging out with members of Chicago's (extremely active) Socialist Party. There are still lingering problems in Jurgis's life that can't be solved: Marija has become a prostitute and a drug addict and all of the children have been corrupted by their life in the city. But he has found a place in a movement that is much larger than him, the movement for workers' rights. It is this political activity that gives him hope for a better future for all of mankind, if not for Jurgis personally.