Jack Holborn is haunted by the memory of the night ten years ago when he lost both his parents at sea. He remembers a ship, the 'Charming Molly' on that fateful night. Jack's dream of becoming a cabin boy on the 'Charming Molly' is nearly achieved when Captain Sharringham offers to take him on board. However, the Captain's twin brother, Judge Sharringham, thwarts Jack's plan. Captain Sharringham plans with pirates to attack the treasure ship 'Esperance', but a traitor reveals the plans to the Admiralty. Captain Sharringham manages to escape but The 'Charming Molly' is captured and returned to Bristol. Jack stows away on her next voyage but is soon discovered.

The 'Charming Molly' is recaptured by pirates led by Captain Sharringham. Jack is allowed to remain as assistant to the ship's cook. Jack strikes a deal with the Captain that if he saves his life three times, the Captain will reveal the secret of what happened to Jack's parents. Their ship, the 'Charming Molly', arrives at Nautia where the pirates are seeking fresh news of the 'Esperance'. While they are on the island the Captain's twin brother, Judge Sharringham arrives. The brothers fight a desperate duel and the Judge Sharringham is shot and left for dead whilst the Captain, impersonating his brother, the Judge, escapes. Jack gets the wounded man aboard the 'Charming Molly', mistakenly believing he is fulfilling his bargain with the Captain.

Sailing from the Island of Nautia in pursuit of the treasure ship 'Esperance' a raft is sighted with a man onboard waving the skull and crossbones. The pirates take him onboard the 'Charming Molly' to discover he is Mr Trumpet, one of the crew of the 'Esperance' who claims to have been a mutineer. The crew of the 'Charming Molly' plan their own mutiny against the man they believe to be their captain. A mutiny is averted by Jack who is warned in time by Mr Trumpet. Still believing Judge Sharringham to be his twin brother Captain Sharringham, Jack claims their bargain is near complete. The 'Charming Molly' encounters the treasure ship 'Esperance', but in the raging seas, the two ships collide.

Marooned on an Island off the coast of Africa, Jack and his fellow survivors dive into the submerged wreck of the 'Esperance' to salvage all her valuable treasure. They share out the jewels but Mr Trumpet claims as his own a gigantic diamond called 'The White Lady', which has a reputation for bringing bad luck. They learn that the man they thought was their leader Captain Sharringham is really his twin, the Lord Justice Sharringham. The Judge states that, as Jack has saved his life three times (completing a bargain made with the Captain) he will keep his brother's promise by helping Jack unravel the mystery of his parent's disappearance. They see a caravan of slave traders and one of their prisoners is Captain Sharringham.

They follow the caravan of slave traders to the slave market where Judge Sharringham is due to be sold at auction. Jack is desperate to prevent the Judge being sold to the vengeful Mr Downes. Downes believes the Judge to be his twin brother Captain Sharringham, who once had him whipped as punishment at sea. Jack gives away his share of the treasure from the 'Esperance' to buy not only the Judge's freedom, but the other slaves freedom from the slave trader Fared. They book passage for Bristol from Anamaboe. Judge Sharringham stops off as the African Island of Nautia to enquire after his twin brother, the pirate. He learns that Captain Sharringham is already in Bristol and is impersonating him on the King's Bench.

They return to Bristol and encounter Captain Cox who now blames all his misfortune on Captain Sharringham and denounces the man he believes to be the Captain to the Harbour Master. Jack does nothing to prevent this happening knowing that the real Captain Sharringham will be forced to try his own brother, Judge Sharringham, the man Cox has mistakenly accused. In the court the deception is exposed. Captain Sharringham is sentenced to be hanged. Jack insists on going to see the Captain in prison. The Captain does not agree that Jack kept to his bargain but gives him another chance if he brings a small silver tube which he can swallow to survive suffocation during the hanging, he will tell him the secret. Jack does this and he is finally told the truth.