Nineteen per cent. That is the turnout in a March by-election-and to King Henry, watching Henry V from the upper gallery of the Globe, it is not a statistic but a diagnosis. A thousand people have paid to stand in the April cold for a four-hundred-year-old play about a king. One in five could be persuaded to choose a government. Henry waited seventy years for a throne he has occupied for four. He spent those decades reading Bagehot and Dicey, filling journals with a question the arrangement forbids him to ask aloud: what is a constitution worth when it rests on nothing but the good faith of the men who operate it? Then the Commons votes no confidence in Prime Minister Alistair Welles-330 to 318-and Welles announces he intends to remain in office anyway. At seven the next morning, a proclamation issues from the Palace under the Great Seal. For the first time since the seventeenth century, a British monarch has dissolved Parliament against the wishes of his government. What follows is not a coup. It is something stranger, and more plausible: a constitutional crisis fought in Orders in Council and Supreme Court filings, in the private arithmetic of the people who actually make a government move. A Cabinet Secretary decides whether her office will act on a piece of paper. A captain in the Household Division opens a sealed order she hoped never to see. Journalist Maya Cho pulls a thread running from an emergency procurement mechanism to a set of Mayfair lunches, and finds that the men cheering loudest for their King have interests of their own. In a flat in South London, Daniel Osei, who did not vote, discovers the constitution has arrived at his door. Across three weeks in April, a decent man with the right diagnosis reaches for the wrong cure-and a country that had stopped paying attention begins, street by street, to pay attention again. Hilary Mantel by way of John le Carré: a debut political thriller about legitimacy, and what it costs to be the last person left who can act.