Phantom Indrajal Comics: Mystry of lost Diamonds:

The incident in the story happened long before the Diana`s marriage. Large ship sank in the Caribbean sea with lots of uncut diamonds valued over 30 lacs due to storm. Pit and Gill two professional thieves have got this information from local newspaper they also informed that local coastal guard will start rescue mission for those lost diamonds within a month. 

Pit and Gill therefore started their own operation to snatched those lost diamonds before the rescue mission, but their plan failed when they realized that  the area was heavily guarded by coastal police. In the mean time Diana Palmer was selected for coming olympic games to represent America in diving and became famous by the newspaper article on her.

Pit and Gill planned to approach Diana for underwater shooting on Caribbean sea. They planned that they will steal those diamonds behind the scene of Shooting with Diana who was famous for her diving talent . At the same time coast guard police will not suspect them.

Diana accepted their invitation after long waiting for Phantom. Diana departed for the shooting before the letter from Phantom came. Pit and Gill kidnapped Diana with the stolen diamonds when she realized their evil plan. Coastal guards also suspect Diana for the slolen diamonds as they locate Diana with the thieves on previous day.

You have to read the full graphic Comics to know how Phantom rescued and cleared the bad name of  Diana from those thieves when he caught them.