While in Africa visiting his friends Mark and Penny Daring, David Curtis helps them thwart attempts to steal the claim papers to a diamond field and to capture Penny as a hostage.
Teenagers Mark and Penny Daring and their friend David Curtis are attacked on a tour of the Parisian catacombs, become separated, and must find their way out alone.
When the Daring family goes to Germany, they are followed by remnants of the former East German Secret Police, and everything climaxes during a chase on the Rhine River.
While in Germany with their father, Mark and Penny Daring and their friend David become involved in trying to figure out how a fortune in jewels was stolen from a well-protected castle.
Penny, Mark, and David thwart the scheme of Hoffmann who attempts to smuggle nuclear weapons from Russia, through Germany's Black Forest, and into Iran.
While sightseeing in Rome, siblings Penny and Mark and their friend David find themselves mixed up with a group of political terrorists and kidnappers who are using counterfeit money to try to frame prominent bankers and government officials.
While scuba diving off the coast of Portugal, Penny and Mark and their friend David discover a gold smuggling ring inside an ocean cave and find themselves in great danger as they try to evade their pursuers through unfamiliar tunnels.
While sightseeing in Madrid, siblings Penny and Mark and their friend David tangle with thieves over a fortune in stolen jewels and trust God to keep them safe from danger.