Sophie, an eight-year-old girl in an orphanage, cannot sleep. Looking out of her window, she sees a mysterious Giant Man in the street, carrying a suitcase and a trumpet. The giant sees Sophie, who tries to hide in bed, but the giant picks her up through the window. Sophie is carried to a large cave in the middle of a desolate land, where the giant sets her down. Believing that he intends to eat her, Sophie pleads for her life, but the giant laughs and dismisses the idea. He explains that although most giants do eat humans, he does not because he is the Big Friendly Giant, or BFG; he had carried Sophie off merely so she would not reveal that she had seen a real giant, which would put him at risk of being captured for a zoo-exhibit.
The BFG explains, in a unique and messy speech, that his nine neighbours are much bigger and stronger giants, who all happily eat humans every night. They vary their choice of destination both to avoid detection and because the humans' origins affect their taste. For example, people from Greece taste greasy, so no giant goes there, while people from Panama taste like hats. As he will never allow Sophie to leave in case she tells anyone of his existence, the BFG reveals the purpose of his suitcase and trumpet: he catches dreams in Dream Country, collects them in jars, and gives the good ones to children all around the world, but destroys the bad ones. Since he does not eat people, he must eat the only crop which grows on his land —-- the repulsive snozzcumber, which looks like a cucumber.
When the Bloodbottler, one of the other giants, enters the cave uninvited, Sophie hides in the snozzcumber; not knowing this, the BFG, in the hope that its revolting taste will drive the Bloodbottler away and thus prevent his discovering Sophie, tricks the Bloodbottler into eating the vegetable. The Bloodbottler takes a bite of the snozzcumber; unknowingly putting Sophie in his mouth. Luckily, the larger giant spits her out unnoticed and leaves in disgust, much to the BFG's and Sophie's relief. They then drink frobscottle, a delicious fizzy drink where the bubbles sink downwards rather than upwards, causing powerful and noisy flatulence, which the BFG calls "whizzpopping".
The BFG takes Sophie to Dream Country but is bullied along the way by his neighbors, led by Fleshlumpeater, the largest and strongest. Sophie watches the BFG catch two dreams—while one would be a good dream, the other is a nightmare. Resentful of the other giants' mistreatment of him earlier that day, the BFG sneaks up to where they are napping and looses the nightmare on Fleshlumpeater, who has a dream about a giant killer named Jack and accidentally starts a brawl with his companions due to his indiscriminate flailing and thrashing about while still asleep.
Sophie persuades the BFG to approach the Queen of England for help with the other giants. She navigates the giant to Buckingham Palace, where he places her in the Queen's bedroom. He then gives the Queen a nightmare that closely parallels actual events; because the BFG setting Sophie in her bedroom was part of the dream, the Queen believes her and speaks with the giant over breakfast. Fully convinced, she authorizes a task force to travel to the giants' homeland and secure them as they sleep.
The BFG guides a fleet of helicopters to the sleeping giants. Eight are successfully shackled, but Fleshlumpeater awakes; Sophie and the BFG trick him into being tied up. Having collected the BFG's dream collection, the helicopters carry the giants back to England, where they are imprisoned in a massive pit.
Every country that the giants had visited in the past sends thanks and gifts to the BFG and Sophie, for whom residences are built in Windsor Great Park. Tourists come in huge numbers to watch the giants in the pit, who are only fed snozzcumbers (although they receive an unexpected welcome snack when three drunks manage to climb the safety fence one night and fall in). The BFG receives the official title of Royal Dream-Blower, and continues bestowing dreams upon children; he also learns to speak and write more intelligibly, writing a book identified as the novel itself, under another's name.
