The Professor eagerly awaits the arrival of a colleague, Professor Doyle, but upon arrival, all are surprised to find that Professor Doyle is a woman. During her stay, the two find some of the older men of the Keewazi tribe to have quite startling ages and learn about a great fountain that may be responsible for their apparent longevity. Professor Porter, who feels he's too old for Robin romantically, secretly sets forth to find this fountain. (thetvdb.com)