The protagonists of Within This Darkness are fourteen-year-old Jeremy Gardela, born in 2004, and his cousin, fourteen-year-old Agata Obara, born in 1900. After an unexpected visit from a Canadian military officer, asking pointed questions about the family history, Jeremy's 70-year-old Aunt Kasia sends Jeremy on a treasure hunt through the forests surrounding Douro, Ontario. Kasia equips Jeremy with maps and journals passed down through generations of the Obara family, and following maps dating from the 1940s, Jeremy discovers the cavern where Agata has lain asleep for a century. Jeremy is able to wake Agata, which her sisters in the early 1900s had not been able to do. He takes her to visit his aunt, and soon they are pursued by the military officer who had questioned Kasia, and who knows about the magical abilities that lie within the history of the Obara / Gardela family.
The action in the book follows Jeremy's search for Agata, and then Jeremy & Agata's search for the second "treasure" which is described in notes and journals given to them by Aunt Kasia. As they search for this treasure, they are pursued by the military officer who had questioned Kasia, who wishes to harness any magical abilities possessed by these children for military use.

Theme
In Howards End, E.M. Forster pens the phrase, only connect. While Within This Darkness is an outdoor adventure story with magical elements, it is also about loss, and the idea that the loneliness and sadness that you carry within yourself can be at least partially alleviated by connecting with others. In Within This Darkness, Jeremy has lost his older sister, and Agata has lost her entire family and even her natural place in history. When these two find each other, and come to trust each other, they begin to move past what they've lost, and start to have hope that they have in fact been found.

Within This Darkness is about loss, and connection.