Quick recap
Actress Jane Wilkinson asks Poirot to talk her husband, Lord Edgware, into a divorce. She wants to marry a duke and says in front of everyone it would be easier if he died. That night Lord Edgware is stabbed at home, and the butler swears he saw Jane arrive. But Jane was at a dinner party of thirteen, with dozens of witnesses. Soon after, impersonator Carlotta Adams dies of an overdose. Poirot works out the trick: Carlotta went to the dinner disguised as Jane while Jane killed her husband. To cover her tracks, Jane also killed Carlotta and a third man. She's arrested and writes to Poirot from prison, without a hint of remorse.
