Tony is consumed by depression.
'Isabella' is one of those episodes that has everything. Great plot, intelligent writing, awesome acting, humour, visual storytelling, and a very effective soundtrack.
This is one of the best screen portrayals of medicated depression. It is translated not just with James Gandolfini's immense performance, but also by the cinematography, editing and music. I could feel the weight on Tony's shoulders and experience his dreamy, outworld perceptions of reality. He looks and feels like the empty house in Dr Melfi's painting: rotted out from the inside. There are some great themes associated with maternal relationships and Tony's existential crisis that play out fantastically in his subconscious.
When the episode does a significant change in tone it is equally as good, just in different ways. We witness one incredibly powerful and suspenseful visual sequence, before the humour and classic character moments fly at us thick and fast.
Other characters contribute fantastically. Carmela, Livia, Junior, Christopher, Paulie, Silvio, Dr Melfi, and even AJ all have memorable scenes. Their respective actors are all spot on with their performances.
This is one of the best episodes of the entire show.