The fascination of Tom for Caravaggio works has, in this episode, the presumed explanation in self identification. The reference of priest from San Luigi di Francesi to light reminds this , like the explanations of guide of Galleria Borghese.
The steps to become Richard - this is the skin of the episode, dominated by admirable scenes like the reaction in hotel room or words about him in the letters to Marge or the evening of Christmas.
The heart of episode - no doubts, the conversation between Marge and Tom, admirable duel and precious clue to discover Dakota fanning as the perfect option for Marge.
In short, the bitterness of desired happiness.