Gretel has wires in her head. Gretel likes to pick wildflowers. Gretel is one of the subjects on the farm, and she is Doctor Gottlieb's responsibility, but she knows something she isn't telling—and if Doctor Gottlieb doesn't figure it out, it may be his body in a ditch next. This story is set in the world of Ian Tregillis' Milkweed series, which began with Bitter Seeds.

It's 1939. The Nazis have supermen, the British have demons, and one perfectly normal man gets caught in between Raybould Marsh is a British secret agent in the early days of the Second World War, haunted by something strange he saw on a mission during the Spanish Civil War: a German woman with wires going into her head who looked at him as if she knew him. When the Nazis start running missions with people who have unnatural abilities—a woman who can turn invisible, a man who can walk through walls, and the woman Marsh saw in Spain who can use her knowledge of the future to twist the present—Marsh is the man who has to face them. He rallies the secret warlocks of Britain to hold the impending invasion at bay. But magic always exacts a price. Eventually, the sacrifice necessary to defeat the enemy will be as terrible as outright loss would be. Alan Furst meets Alan Moore in the opening of an epic of supernatural alternate history, the tale of a twentieth century like ours and also profoundly different.

Someone is killing Britain's warlocks. Twenty-two years after the Second World War, a precarious balance of power maintains the peace between Great Britain and the USSR. For decades, the warlocks have been all that stand between the British Empire and the Soviet Union—a vast domain stretching from the Pacific Ocean to the shores of the English Channel. But now each death is another blow to Britain's security. Meanwhile, a brother and sister escape from a top-secret research facility deep behind the Iron Curtain. Once subjects of a twisted Nazi experiment to imbue ordinary humans with extraordinary abilities, then prisoners of war in the vast Soviet effort to reverse engineer the Nazi technology, they head for England. Because that's where former spy Raybould Marsh lives. And Gretel, the mad seer, has plans for him. As Marsh is drawn back into the world of Milkweed, he discovers that Britain's darkest acts didn't end with the war. And as he strives to protect Queen and country, he's forced to confront his own willingness to accept victory at any cost.

12 May 1940. Westminster, London, the early days of World War II. Again. Raybould Marsh, one of "our" Britain's best spies, has travelled to another Earth in a desperate attempt to save at least one timeline from the Cthulhu-like monsters who have been observing our species from space and have already destroyed Marsh's timeline. In order to accomplish this, he must remove all traces of the supermen that were created by the Nazi war machine and caused the specters from outer space to notice our planet in the first place. His biggest challenge is the mad seer Gretel, one of the most powerful of the Nazi creations, who has sent a version of herself to this timeline to thwart Marsh. Why would she stand in his way? Because she has seen that in all the timelines she dies and she is determined to stop that from happening, even if it means destroying most of humanity in the process. And Marsh is the only man who can stop her. Necessary Evil is the stunning conclusion to Ian Tregillis' Milkweed series.