The Diamond Cup of Alexander is a treasure of immeasurable price, a drinking cup fashioned out of a single, enormous diamond. It was originally made for Alexander the Great. Like many of the rare and historically important articles in the treasure-room, it is regarded by the Phantom not as a possession but as a trust for safekeeping, to be guarded for the sake of posterity against the ravages of nature and the corruption of men.

It came into the Phantom's possession in the 1600's. At that time, it belonged to the Sultan Mamoud ben Al'Lina, one of the world's most powerful men. The Phantom saved him from murdering bandits, and the Sultan offered to repay the Phantom with any item from his treasure.... except the Diamond Cup. That was a betrothal gift, which he asked the Phantom to deliver to the Sultan's intended bride, the Princess Pura.

Phantom accepted the perilous assignment. When he finally arrived at the maharajah's palace he fell in love with the Princess, but presented the cup in the sultan's name, anyway. The princess refused it because she did not love the old sultan and would not marry him. But the maharajah insisted that the Phantom take her to the sultan anyway, unaware of the Phantom's feelings for her.

The journey back was fraught with near-disaster. In the midst of it all, the princess revealed to the Phantom that she was desperately in love with him, but he was honor-bound and wouldn't take her away as she begged. She even tried to throw herself off a cliff in despair.

When they reached the sultan, the Phantom refused to turn over the princess, and offered to return the Diamond Cup. He was arrested. The sultan had him placed in an arena with his sword and his horse, matched against three man-eating tigers, so Pura could watch his death. Instead, she leapt into the ring to die with her love. The sultan had the tigers shot, and the Phantom grabbed the princess and rode off with her.

When the sultan realized the truth of the situation and saw the strength of the princess' love for the Phantom, he forgave them, and presented them with Diamond Cup of Alexander as a wedding present.