A Story Within a Story. "Tamar"

Tamar by Mal Peet is an book that tells two stories at once. This book tells of espionage, passion and betrayal of both stories.
The first story sets in modern day on the perspective of a young girl named Tamar. Her name Tamar was given by her grandfather who recently committed suicide who he also left a box to her.
The other story sets in the period of late World War Two more then 50 years earlier. Two agents from Special Operation Executive were sent by England to Netherlands. Their task is to unite the local resistance against Nazi's occupation. The soldiers was known by their code name, Tamar and Dart. As the young girl Tamar further uncover the mystery behind the mysterious box her grandfather left her, the more story of Tamar from the past is revealed.
This is a new book I started reading. The young girl from the modern is definitely relate to the Special Operation Executive agent's story in a way. My personal guess her grandfather is probably the agent code name Dart, who was guilty (base on the "Betrayal" from the introduction) to what he did. That might explain modern Tamar grandfather's suicide. I wonder how is this going to change the girl's life, who don't seem to have any physical connection to the story happened 50 years ago.