The year is 1500. Leonardo da Vinci is 48 years old and has been forced to leave Milan - his home for the past 18 years. He travels east, towards Venice, a city under threat of invasion from the Turkish fleet. Leonardo suggests the Venetians implement his idea for an underwater army - men in scuba gear who will attack from underwater. The Venetians remain unconvinced and never put his ideas to the test. 500 years later we take up Leonardo's designs and, with the aid of diving experts, put them to the test in the Venetian lagoon. Presenter Alan Yentob trails Leonardo in his return to Florence, the city of his youth, following rejection by the Venetians. We find him absorbed in his scientific research, particularly anatomy. He's also seen drawing the famous Virgin and Child with St. Anne. Leaving Florence in 1502 we find him in the company of the notorious Cesare Borgia. Alan visits Imola, a town which Leonardo mapped out for Cesare, creating a unique and prized map of the time. ...