'A Splendid Sin' is a love story. It is fiction based on fact.
It describes the love of an older man for a younger man and how this love helps create a great work of art, 'The Last Judgement' which stands on the altar wall of the Sistine Chapel. It is also the story of genius expressed in artworks breaking boundaries and pushing art into a new form (Mannerism).
The forbidden, intense Renaissance same-sex romance takes place under the noses of a powerful Vatican elite headed by the Pope. One man is an adoring muse of the artist and the other, the artist Michelangelo, can only see the beneficence of God in the beauty of his muse. The concept of active same sex love was condemned by Church teaching, much like today.
The genius is a forward thinker, he expresses his innermost feelings in the art forms he creates, mostly unconsciously, but the artworks reveal his soul. His admiration of the perfect human male form and the expression of his sexuality causes him to be conflicted and tortured, yet it is part of his artistic soul in which he loses himself to create. He is a lone genius tortured by his own piety as he breaks rules concerning art and the society he lives in.
This creates jealousy, disturbance, paranoia, vindictiveness and betrayal by those who wish to bring Michelangelo down. Many of those attached to the Vatican can only see his art as lewd. His relationship with Tommaso de Cavalieri is an expression of all these elements and through the progression of this story he changes until his soul is quietened and he loses himself, not in the love of Tommaso, but in the love for his faith.