Cornwall was being terrorised by a giant named Cormoran. He lived with a cave on Mount Cornwall and would wade to the mainland to steal what he wanted. The townspeople decided that the reward for the person who killed the giant was his treasure. Jack, a farmer's son, volunteered to undertake the task.
One dark night, he dug a deep pit beside the mount. He called to the giant, who came out and fell into the pit. After teasing the giant for a while, he knocked Cormoran on the head with the pickaxe, and killed him. After this day he was known around town as Jack the Giant-Killer. The townspeople presented him with a sword and belt with letters of gold on it, naming him as Cormoran's killer.
News of Jack reached another giant named Blunderbore, who vowed revenge on Jack. This giant lived in an enchanted castle in the middle of a forest. One day, Jack was walking near this wood while on his way to Wales. He took a nap beside a small fountain. Blunderbore found him there, and recognising him by the letters on his belt, carried him to the castle. After the giant locked Jack in a chamber, he left to find his brother.
There were ropes in a corner of the room Jack was locked in. When he spied the giants coming back, he took the ropes and made them into nooses, and, throwing the ropes over each giant's head, he drew the other ends over a beam and pulled hard, throttling them. He proceeded to open all the doors in the castle, and found three ladies, which he freed. Then he continued on his way.
Jack travelled as fast as he could to make up for lost time, and soon became lost. He met a Welsh giant with two heads, who made a great show of friendship. The Welsh giant agreed to put him up for the night, but Jack overheard him later, planning to knock his brains out. Jack placed a pillow in the bed instead, and the next morning, when the surprised giant asked him if his sleep had been restful, he said yes and mentioned a rat had run over his head a few times.
For breakfast, the giant brought Jack a huge bowl of porridge. Jack tied a large bag below his chin, and emptied the porridge into it. Telling the giant that he had a trick to show, Jack took a knife and slit the bag open, letting the porridge run out. The giant decided he could do the trick to, and taking the knife, he slit his belly open and died.
Jack, upon hearing of the generosity of the king's son, desired to become his servant. One night in Wales, they were without food or shelter. Jack told the prince of his uncle's house a few miles away. His uncle was a giant with three heads and could fight five hundred men in armour. Jack persuaded the prince to let him go ahead and prepare the way.
Jack knocked on the door, and told his uncle that the prince was coming with a thousand men. His uncle was afraid and ran and hid in a vault, asking Jack to lock him in. Early next morning, Jack gave the prince a good supply of money and sent him on ahead. He let the giant out, and took as his reward a coat of invisibility, a cap of wisdom, a sword that would cut anything, and a pair of seven league boots.
Jack ran and joined the prince, who was at the house of a lady who was possessed by seven evil spirits. At the end of dinner, she wiped his mouth with a handkerchief and put it in her bosom, telling the prince that he was to produce the handkerchief the next morning or lose his head. In the middle of the night, she went to the devil, followed by Jack in his seven league boots and coat of invisibility. She gave the devil the kerchief, and left, and Jack stole the kerchief back, and also left.
The next day, the lady kissed the prince, and told him that he must show her the lips she kissed last night, the morning after. That night, she went to the devil and kissed him. After she left, Jack cut the devil's head off, and the prince showed her the severed head the next morning. This broke the enchantment, and the evil spirits left her.
Jack kills several more giants, before getting married to the daughter of a duke (who, incidentally, had been kidnapped by a giant). She must have been a settling influence, because they lived happily ever after in a castle that the king gave them.