Family and community
The major tension in the novel is that it is difficult for the Williams family to handle the truth about Eunice, emotionally speaking. She was forced to make horrifying decisions under duress, and although it was not her fault, she eventually acquiesced. She became a Native person by culture, and she was married to a man and raised a family. For the Williams, that must have looked like a betrayal of the family, but it wasn't—Eunice needed to integrate herself into community in order to survive. But, nothing can change her family's love for her, and hopefully she understood that deeply later in life when she finally reconnected with Stephen.
Love as a relentless pursuit
When Stephen takes up his father's mission of finding Eunice and returning her to the family, he is accepting a calling, to love Eunice relentlessly by pursuing her release, even after she has already forgotten English, even when she has had children. Eunice ignores him for years, but eventually, they reconnect later in life, and although their relationship was stolen from them, in a way, Stephen is rewarded for his time and effort, and there is a sense of communion in their final years.
Death and life
Unfortunately, the story is a true story about how war ravaged a community. Not only were the Native people continually oppressed by the Europeans, the Europeans were also continually oppressed by the Natives. The intensity of this story comes from the fact that so much of the story is literally life and death. Two children were killed, taken from their mother and murdered in front of her, and then those people ended up being Eunice's family and friends, so the intensity in the plot comes from mortality and the heartbreaking travesty of human death and warfare.
