The Conservative Party has been in power for 47 of the 65 years since the end of World War II. During that time the division within the party over Europe has been the enduring drama of British politics--from Churchill's decision not to join the original European Coal and Steel Community in 1951 to Cameron's decision to hold the Brexit referendum in 2016. Other leaders came and went, but the issue was always there--sometimes center-stage, at others behind the scenes--destabilizing foreign policy, corroding the body politic, and destroying several of the party's leaders. These questions, and how they panned out, created a deep, grumbling discontent--the worm in the apple--that, over time, turned the Conservative Party and, by extension, a significant section of the electorate, against British membership of the EU. By telling the story of the arguments and divisions within the Conservative Party, The Worm in the Apple helps to explain why Britain voted to leave the European Union in 2016.