Darwin's life is a bit of a mystery, and John Darnton took the known facts of his life and let fiction build the rest. He coupled it with some modern-day sleuths that have their own mysteries.
I especially enjoyed Lizze Darwin's time line - we see her as girl near womanhood snooping through Darwin's belongings and then later as a woman in her 20s, unraveling the last strands of her father's mystery.
We also get to see Darwin as a young man traveling the world, hunting and hiking and swimming and collecting.
Overall, very enjoyable historical fiction.