Siberia, 1939. Pregnant with her first child, Lena Orlova plans a daring escape from a brutal Soviet prison camp to the one place she knows is safe: a cave containing the legendary Altar of Bones, hidden behind a frozen waterfall deep in the icy wilderness. San Francisco, Present Day. Zoe Dmitroff discovers that she is the last in a line of women who have been entrusted with a secret so great many have died preserving it. Propelled into a dangerous quest to discover exactly what she was born to