The Knowledge is the test that prospective London cab drivers must pass before they can receive their 'green badge' or permit, and involves memorising every street and public building within a six-mile radius of Charing Cross station. Urged into applying by his girlfriend Janet, Chris attends an acceptance interview at the Metropolitan Police Public Carriage Office with nine other candidates, among them Ted, Gordon and 'Titanic'. The intimidating chief examiner, Mr Burgess, prepares them for the daunting process ahead by presenting each of them with their Blue Book, which lists 468 'runs' or routes between a specified source and destination that they must successfully commit to memory and recite at regular testing sessions. He predicts that seven of the ten will never succeed in becoming a taxi driver, and warns against obtaining any criminal conviction, since this will automatically disqualify them from the process. Ted, who wants to follow in the family tradition of being a cab driver, tries to ingratiate himself with Burgess, but is coolly rebuffed.

The four set about trying to learn the first run: Manor House Station to Gibson Square. Chris, who is on the dole and doubtful of his ability to succeed, is goaded into action when Janet presents him with a moped to practice his runs. Gordon's wife, Brenda, is unhappy about the effect his enforced absences will have on their marriage and her free time, whereas Ted's wife, Val, gives him unstinting support. Titanic's wife has not spoken to him for 17 years, and appears not to even notice him cycling uncertainly off on his runs. Chris discovers Gordon abandoning one of his runs for a tryst with a married woman, and it eventually becomes clear that she is not alone in enjoying his attention. When Ted leaves the marital bed in the middle of the night, however, it is only to practice some runs when the roads are quiet.

Two months after the acceptance interview, the 'Knowledge Boys' attend their first 'appearance', a testing session conducted by Burgess, now nicknamed 'The Vampire'. This is a gruelling experience since not only do they have to recall the runs, but Burgess tries his utmost to distract, unsettle and aggravate them. Ted, despite being blessed with a photographic memory, loses his temper; Titanic's mind goes blank; Chris, physically sick from nerves, rings from a phone box to reschedule the appointment. For Chris, the Knowledge has become an all-consuming obsession, whereas Gordon admits that his extra-marital affairs are carried out on two of the evenings he should be practicing his runs. Brenda tolerates his infidelities as long as he ultimately succeeds in earning a respectable wage as a taxi driver.

Gordon and Brenda invite the other three couples to tea, although Titanic attends without his wife. The four men can talk of nothing but the Knowledge, and the three women struggle to have a conversation together. The others are shocked when they realise how far ahead of them Ted is, and a disheartened Chris resolves to see less of Janet, much to her chagrin. Ted is naturally the first to obtain his green badge, and is cajoled by Chris and Gordon into celebrating with champagne, despite rarely drinking alcohol. After leaving the pub on his moped, Ted is stopped by the police and breathalysed. The charge of drink driving means the removal of his green badge, and disqualification from re-applying for twelve months.

At one appearance, Gordon finally loses his patience with Burgess, who throws him off the Knowledge for not being able to control himself. Chris's relationship with Janet deteriorates and she breaks up with him in favour of a work colleague. Ted decides to emigrate to Israel with his family, and Gordon is able to continue his affairs as a milkman. Only three of the original ten manage to get their green badge: Chris, Titanic and the only female candidate, Miss Stavely. Titanic leaves his wife, much to their mutual relief. Almost three years after the acceptance interview, Chris becomes a fully licensed taxi driver, and soon after is incredulous when a customer requests an obscure route: Manor House Station to Gibson Square.