Steampunk alternate history action-adventure! He even expands the point of divergence in the appendix, as it's an unusual one.
The opening does some weaving back and forth from the present to the past, about Tim on the steampunk submarine of the title, and Clara is snuck out of her school by her mother, only to be captured by Menshevik agents and have to escape an airship. More tangles about how they land on the Cuttlefish.
Subsequent adventures include accusations of theft, a canal, changes of plan, the question of what Clara's grandmother researched, Clara's imprisoned father, the flooding of London, and more.