The third and final book of The Milkweed Triptych.
Necessary Evil contains examples of:
- Alternate History: Another alternate history - an in-universe alternate timeline from the alternate history of Bitter Seeds. In this one, Gretel makes sure the war mostly plays out like in our history.
- Bittersweet Ending: The warlocks are dead and humanity is saved. Marsh-2 gets to live out his life with Liv and their daughter. Marsh-1 however has to go into hiding alone overseas as he's a wanted man, and Will has been murdered by Gretel.
- Brought Down to Normal: Gretel, and twice to add insult to injury; once, when the interference of two timelines stops her predicting that Liv would be rescued from the bombing of Coventry, and finally when she has her wires removed by both Marshes at the end of the story.
- Character Tics: Marsh1 eventually gives himself away to Will by his habit of cracking his knuckles against his jaw.
- Dramatic Drop: Gretel scalds herself with boiling water when she drops a kettle the first time her prediction goes wrong.
- Historical In-Joke: Gretel is apparently responsible for the Germans' slow response to the Dunkirk evacuation, and for otherwise making sure the Second World War actually happens in the way we remember it happening.
- I Hate Past Me: Marsh becomes disturbed when meeting his younger self and realizes how much of a self-righteous abrasive Jerkass he was and how this is how people see him.
- I Want My Beloved to Be Happy: Marsh1 forgoes life with Olivia so that Marsh2 will have the happy marriage they never had.
- This Cannot Be!: Gretel suffers a Villainous Breakdown on discovering Liz survived the bombing of Coventry.
- Multiple Narrative Modes: Most of the book is in third-person, but Marsh1's POV sections are in first-person. This helps to avoid confusing the two Marshes...
- My Future Self and Me: 2 × Raybould Marsh.
- Nice Job Breaking It, Hero:
- Marsh1 misses a chance to contact Will because Will turns up at the police station in response to a call that his billfold has been stolen, tells the police it must be a practical joke (as he still has his 1940's billfold), then leaves without even looking at the mysterious stranger who was caught with it. His Milkweed boss tears strips off him for this when it's later assumed that Marsh1 is a German spy.
- Marsh1 later sleeps in, missing his second and last chance to intercept Will before he recruits the warlocks.
- Pay Evil unto Evil: Gretel's exile; the island is too windswept to allow wildflowers to grow (Gretel's hobby), and the last words of both Marshes are to inform Gretel that her brother Klaus is dead.
- Reality Warper: The Eidolons. Shown in detail when Will summons an Eidolon for Marsh2 in Berlin. After 7000 heartbeats in close proximity to an Eidolon, Marsh2 starts losing his grip on how reality actually works.
- Ripple Effect Indicator: When Marsh1 tries to shoot Marsh2 in the knee and fails, their knee pain suddenly goes away.
- Sealed Room in the Middle of Nowhere: At the end of the book Gretel is abandoned on a tiny windswept island off the Scottish coast, with her food being resupplied every six months.
- Set Right What Once Went Wrong: Gretel's plan was to create a timeline in which the Eidolons do not kill her or humanity.
- Tele-Frag: Marsh1 kills the twins this way.
- Time-Travelers Are Spies: Marsh1 ends up in wartime London with another man's wallet and no period currency. Naturally, he's suspected of being a German spy. Fortunately he's then released in a Trick-and-Follow Ploy. As he really is a spy, but a British one and so familiar with London, he's able to evade his followers.
- Trust Password: Needed to enter Milkweed. Fortunately for Marsh1, they're the same as in his timeline.
- The Uriah Gambit: Marsh1 steals the blood samples of the warlocks and puts it in the box used for the teleportation, so the warlocks will be unexpectedly transported (along with the commando team) to the war zone, where he can more easily arrange their demise.
- Villainous Breakdown: Gretel when she realizes that she no longer has a perfect view of the potential timelines.
