S1·E10 Tarzan and the Fountain

The Professor eagerly awaits the arrival of a Doctor Doyle. Upon arrival, everyone is surprised to find that Doyle is actually a woman. Doctor Robin Doyle had come to Africa to study Keewazi's tribe with the Professor. While studying, they find some of the older men to have quite startling ages (from 500 to 700), and learn about a great fountain. The professor, who feels he's too old for Robin, secretly sets forth to find this fountain.