Filippo has never met his father. Before he was born, his father, a jeweller, travelled to the court of the Great Moghul Emperor Shan Jehan. Twelve years later, a stranger, Signor Khan, arrives bringing a message. His father is held captive by the bandit Mir Baba and only the most valuable jewel, his masterpiece The Ocean of the Moon is worth enough to raise the ransom. Filippo must go with the stranger, into the foreign lands, to sell the jewel to Shah Jehan. But he goes with a secret: he is carrying a copy of the jewel, the real diamond is sewn into his scalp, beneath his hair. They are attacked almost as soon as they set out on their journey. The fake jewel is taken, and Filippo discovers Khan has betrayed him - he knew all along about the real jewel. They travel on to the Emperor's court, where Khan tries to force Filippo to sell the jewel to the Grand Vizier, and Prince Aurangzeb puts him under house arrest. But when Filippo manages to meet Shah Jehan, the Emperor is delighted with The Ocean of the Moon and the plan of the Grand Vizier, in the pay of Aurangzeb, is thwarted. But these are dangerous men who will not give up.
As Filippo and Khan travel into Afghanistan, bandits in the pay of Aurangzeb attack, kill Khan and Filippo flees for his life. Reaching the bandit stronghold, Filippo can only trade a small ring for his father, but it is enough. But worse is to come. His father has been driven mad by his captivity and dies on the journey home. Filippo is embittered, but finds his sadness healed when, years later, he returns to Hindustan with his brother. The Taj Mahal is a replica of The Ocean of the Moon, their father's work has not been in vain.