Eddie Asher arrives at Hudson Valley Psychiatric Hospital panicked that he may have murdered his brother's fiancée, Lucy, with whom he shared a profound kinship. He can't imagine doing such a terrible thing, but Eddie hasn't been himself lately.

Eddie's anxiety is nothing new to Pär, the one Eddie calls his Other, who protects Eddie from truths he's too sensitive to face. Or so Pär says. Troubled by Pär's increasing sway over his life, Eddie seeks out Dr. Richard Montgomery, a specialist in dissociative identities. The psychiatrist is Eddie's best chance for piecing together the puzzle of what really happened to Lucy and to understanding his inexplicable memories of another man's life. But Montgomery's methods trigger a kaleidoscope of memories that Pär can't contain, bringing Eddie closer to an unimaginable truth about his identity.