The story takes place in 1825 in London, England.  The main character in this story is Tom Tin. He is a fourteen year-old boy who lives with his mother and his father. When Tom’s dad, an unemployed ship’s captain, is taken to prison because of an unpaid debt, Tom must go out and find money for his family.    Tom’s dad agrees to work off his debt, serving as captain on a prison ship.

            Tom finds a huge diamond in the river.  He promises Mr. Goodfellow that he will repay his father’s debt.  Tom is robbed and loses the diamond.  In a case of mistaken identity, Tom is arrested and convicted of murder.  Tom is sent away on a ship called Lachesis.  He is confused when the other convicts claim to know him.  He makes only one friend on the ship, a small boy named Midgeley.

            Unable to help his father, Tom and Midgeley devise a plan to escape from the ship.   They are caught by guards and are transferred onto another ship that holds juvenile convicts.  The captain of that ship happens to be Tom’s father.  They are reunited and father signs Tom on as  part of the crew.  Father and son will work for Mr. Goodfellow until Tom serves his time or until they can figure out a plan.

            Mr. Tin tells Tom that Tom was once a conjoined identical twin. Mr. Tin could  not raise two children because he was poor. This explained why convicts recognized him.  Tom had been arrested for crimes his brother had committed!