Kara the Jungle Princess

Kara was formerly Jane Howell, an army nurse who was flying a native African named Boldayo back to his people on a plane piloted by Major Christopher "Kit" Walker.  Something went wrong with their plane, and just before it crashed into the Zambezi River, Baldayo gave Jane and Kit a miraculous powder which gave them the ability to survive life-threatening accidents.  Baldayo was killed in the crash, but Jane and Kit stumbled from the wrecked plane unharmed.  They had landed in Arohitia, a country under the water where the Arohitians, a race of immortal warriors, lived.  The Arohitians welcomed them with open arms in the belief that Jane was Kara, their long-lost princess.  A usurper had taken their throne, and they had apparently had to send Kara to safety in the upper world to protect her from the tyrant.  Kit and Jane worked with the Arohitians to destroy this usurper, after which Jane took the throne of Arohiti and took Kara's name, posing as her to make sure the Arohitians were kept safe.  Later, another dose of the powder rendered Kara immortal just in time for her and kit, who was now her guardcaptain, to face Targala, an ancient warrior-priest of the monkey god Esh-Kar, who wanted to make Kara his mate.