The Dead in Their Vaulted Arches

This is about the 5th or 6th in the Flavia de Luce series by author Alan Bradley. Flavia is 11 years old and quite a chemistry whiz, thanks to an inquiring mind and access to her Uncle Tarquin's lab and notes. The supplies and equipment in the lab never seem to run out, even though it has been locked up for ages;  Flavia's particular passion is understanding poisons. Her companions are a chicken named Esmeralda and a trusty bicycle named Gladys. And then there are her two horrible older sisters, Daphne and Ophelia. Father is kind of a non-entity in Flavia's life, interacting with the children only rarely, and then  in a detached way. Partly this is due to the fact that his wife, and his daughters' mother, Harriet, has been missing for 10 years. But now she has been found...apparently the victim of an accidental fall while climbing in the Himalayas. Flavia is determined to try out the research suggesting that the dead can be brought back to life, but is interrupted in her efforts by the arrival of the pathologist from the Home Office. Now, she must live with the fact that Harriet is dead for good. She is jealous that Harriet left when she was just a baby and so both her sisters have more memories of her. But, while rummaging in the attic, Flavia has found some undeveloped movie film featuring her mother, and when she is finally able to view it, has more questions than answers.
     Add this to the strange goings-on at the train station when her mother's body came home: Winston Churchill shows up and talks to Flavia about pheasant sandwiches; a tall stranger seeks her out to deliver a cryptic message to her father; and that same man is subsequently pushed under the train. Moreover, she is beginning to believe that her mother's death was not an accident at all. Trying to wade through the layers of grief at her decaying family home, Flavia is learning all kinds of new information about her mother and father's past. And finally, it seems, Flavia may be heir to something she never expected--service to her country.