Two nuns ask the Minus Five team to help them recover a sacred chalice lost in a wreck off the Great Barrier Reef, but Captain Ted King reluctantly declines assistance as the Endeavour is under orders for another assignment. The nuns risk their lives when they decide to attempt the salvage job themselves.
 

Tense moments mix with comedy when three students capture the Endeavour and, thinking the crew are spies, plan to hand them over to the police.