An exhausted young woman staggers into a provincial workhouse in the middle of the night, gives birth and dies. In the absence of any identification her newborn son is arbitrarily named 'Oliver Twist' by Mr. Bumble, the negligent parish beadle ('a fat man, and a choleric one... [with] a great idea of his oratorical powers and his importance'). Bumble promptly places him in the care of the ill-tempered Mrs. Corney and the drunken Mrs. Mann in the workhouse orphanage, where he grows up 'without the inconvenience of too much food or too much clothing'.
When he reaches the age of nine he is transferred to the workhouse proper and set to work. Drawing the short straw in a lottery amongst his starving fellow inmates, it falls to Oliver to ask for more gruel, the staple diet of the workhouse. As a result he is put to a trade: after narrowly escaping employment as a chimney sweep's boy, he is placed under the care of Mr. and Mrs. Sowerberry. They run an undertaking business with the assistance of the snivelling bully Noah Claypole ('a large-headed and small-eyed youth, of lumbering make and heavy countenance') and their vindictive maid, Charlotte. When Noah insults his mother, Oliver attacks him. Bumble's analysis is as follows: '"It's not madness, ma'am", [he replied] after a few moments of deep meditation. "It's meat."' As a result of the cruel punishment that follows, Oliver flees.
Arriving in London, he is immediately befriended by an urbane, cheerful boy named Jack Dawkins, otherwise known as the Artful Dodger- 'one of the queerest-looking boys that Oliver had ever seen'. The Dodger takes him to the squalid lair of Fagin, 'a very old shrivelled Jew whose villainous-looking and repulsive face was obscured by a quantity of matted red hair'. Fagin decides to apprentice the hopelessly naïve Oliver to him as a pickpocket.
Having learned the necessary tricks, Oliver is sent out with the Dodger and his cohort Charley Bates to ply his new trade. Horrified by the sight of their pick pocketing a gentleman at a bookstall, Oliver flees but is pursued as the culprit and brought before a magistrate. However, the gentleman, Mr. Brownlow ('a very respectable-looking personage, with a powdered head and gold spectacles'), witnesses the flight of the Dodger and Charley. He is thus able to exonerate Oliver, who arouses sufficient pity in him to be taken to his house, where the kindly housekeeper, Mrs. Bedwin, nurses him through a fever. He is treated with a consideration and sympathy hitherto unknown to him, and is happy for the first time. In Bedwin's sitting room hangs a portrait of a woman of which Oliver is 'a living copy. The eyes, the head, the mouth; every feature was the same'. The juxtaposition causes Brownlow to start violently, but nothing is made of it at the time.
Meanwhile, Fagin is desperate for Oliver's safe return, as he is a potential informer. To this end he recruits the vicious robber Bill Sikes ('a stoutly-built fellow of about five-and- thirty... [with] a beard of three days' growth and two scowling eyes; one of which displayed various parti-coloured symptoms of having been recently damaged by a blow') and his pathetic mistress, Nancy. They are swiftly successful in their search for him, and he is expertly kidnapped whilst on an errand for Brownlow. Back in Fagin's clutches, Oliver is sent with Sikes to assist at a break-in in the countryside. Detected whilst effecting his entry, Oliver is shot and Sikes escapes. The householder, Mrs. Maylie, accepts Oliver's fantastic story and takes him in. He is treated with exceptional kindness by her and her beautiful niece, Rose, who is 'in the lovely bloom and spring-time of womanhood... she was not past seventeen... so mild and gentle; so pure and beautiful'.
This idyll is threatened when Rose develops a sudden illness. Mrs. Maylie's strapping son Harry arrives and implores Rose to marry him on her recovery. She refuses him because she is ignorant of her antecedents, having been adopted from a workhouse orphanage (or 'baby-farm'). With the help of the Maylies' friend Dr. Losberne, Brownlow is traced. Whilst staying with the Maylies Oliver catches terrifying glimpses of a sinister man spying on him; he turns out to be Monks, an agent of Fagin. Fagin himself is observed spying on Oliver but disappears without trace, thereby acquiring a diabolic aspect- 'there were not even the traces of recent footsteps, to be seen'. Back at the den, Nancy overhears Fagin and Monks plotting to deliver Oliver once more into Fagin's hands. Overcome with pity, she reveals the details to Rose in a clandestine, emotionally-charged interview ('"Lady! Dear, sweet, angel lady... Do not mind shrinking openly from me!"' invites Nancy; '"I pity you! It wrings my heart to hear you!"' counters Rose). Rose and Losberne eventually promise Nancy that Fagin and Sikes will be immune from prosecution in the event of the enigmatic Monks's arrest.
Fagin, meanwhile, has a new recruit: Noah Claypole, who has absconded from the Sowerberrys with Charlotte and such money as he could steal. Claypole is ordered to spy on Nancy, who has aroused Fagin's suspicion. He follows her and eavesdrops on a meeting she has with Brownlow and Rose on the steps of London Bridge. He faithfully recounts this to Fagin, who duly relays it to Sikes; enraged by Nancy's presumed treachery, Sikes savagely clubs her to death in the novel's most lurid scene ('he beat [her] twice with all the force he could summon... she staggered and fell: nearly blinded with the blood that rained from a deep gash in her forehead... she breathed one prayer for mercy to her Maker'). Sikes promptly flees to the countryside, where he tries to kill his trusty dog Bullseye. When he returns to the gang's headquarters he is treated with revulsion by such gang-members as have escaped a police raid in which Fagin and Noah have been arrested ('"Don't come nearer me!" answered [Charley]... looking, with horror in his eyes, upon the murderer's face. "You monster!"'). Bates raises the alarm and Sikes attempts to escape across the rooftops, but slips and inadvertently hangs himself. Bullseye leaps to his death in an attempt to join Sikes. Following a sensational trial Fagin is condemned to death for unspecified crimes- 'Guilty. The building rang with a tremendous shout, and another, and another, and then it echoed loud groans, then gathered strength as they swelled out, like angry thunder'- and the Dodger is cheerily transported. Noah escapes justice only by testifying for the prosecution.
Back at the Maylies', Brownlow takes custody of Oliver and reveals all. It transpires that Monks is in fact Edward Leeford, Oliver's half-brother. Their father seduced and promised to marry Agnes Fleming, Oliver's mother, whilst still married to Leeford's mother. Their father's will stipulates that Oliver can only inherit his half of the inheritance if he maintains a spotless reputation- hence Monks's efforts to discredit him. Monks emigrates with his half of the inheritance to America, where he dies in prison. Bumble, now married unhappily to Mrs. Corney, is implicated in the plots against Oliver and therefore reduced to dependency on the cruel workhouse system that he so zealously enforced on Oliver ('deprived of their situations [they] were gradually reduced to indigence and misery'). Finally, Brownlow reveals that Rose and Oliver's mother are sisters, and that Rose is therefore legitimate. As a result she can marry Harry, who abandons a burgeoning political career in order to become a country parson at Rose's behest, and raises a memorial to Agnes in his churchyard. At the end we are told that everyone is 'truly happy'.