He took a lifelong vow of poverty and begged for his daily bread, yet he spent his days working with sheets of pure gold and ultramarine pigment more expensive than gold itself. His patrons were the wealthiest, most powerful people on earth, the Medici dynasty and two popes.

This episode traces how a young painter named Guido di Pietro became Fra Angelico, the humble Dominican friar who accidentally became one of the most important pioneers of the early Renaissance. It is the story of balancing your deepest values against the flashy, high-stakes demands of the people who pay you, told through frescoes, mathematics, and a treasure hunt that ended in an Oxford guest bedroom.

• Why ruthless banker Cosimo de' Medici funded a monastery for beggars and kept a personal monk's cell there
• How Angelico abandoned flat gold Gothic back ...