Episode Title:The Fountain.Episode Description:The Professor eagerly awaits the arrival of a colleague, Doctor Robin Doyle, who has come to Africa to study Keewazi's tribe with the Professor, but upon arrival all are surprised to find that Doctor Doyle is a woman. During their study, they find some of the older men to have quite startling ages (from 500 to 700) and learn about a great fountain that may be responsible for their apparent longevity. Professor Porter, who likes Robin but feels he's too old for her, secretly sets out to find this fountain.