Independently intelligible final installment--Tregillis provides an ingenious summary while getting things under way--of the Milkweed Triptych (The Coldest War, 2012, etc.). In this bleak fantasy, World War II was fought between Nazis with devastating psychic powers and British warlocks employing Eidolons, irresistible demons beyond time and space--a struggle the British ultimately lost. In 1963, after the Soviets defeated the Nazis and took over their horrid experiments, Gretel, a psychic so powerful that she can not only view possible futures, but actually create them, escaped from Soviet captivity and fled to England. So terrified of the Eidolons was Gretel--they come to dominate and ultimately destroy reality--that she determined to create the only possible future free of the demons by conveying banged-up former spy chief Raybould Marsh back to an alternate 1940 where the war is in its infancy. Marsh, who fears the Eidolons more than he loathes the fiendish Gretel and doubts the possibility of outwitting her, cooperates while scheming to somehow defeat her and achieve his personal goals: to save his marriage and preserve the life of his infant daughter. Marsh, then, without revealing his identity, must guide his younger self and youthful colleagues along the necessary path. But can he resist the temptation of stealing his beloved wife away from--himself? This time, Tregillis gets many details of 1940s London wrong, though not fatally; what's more troubling is that one of the issues on which the plot depends probably doesn't work. Still, even this doesn't significantly detract from the intensity of the narrative, the torments of the protagonist or the deviously alluring storyline. Darkly fascinating, flaws and all: A thoroughly satisfying conclusion to an imaginative tour de force.