Inspector Japp informs Poirot of the murder. It turns out that Jane arrived at Regent Park the night before, announced herself to the butler and was spotted entering her husband's study by the secretary. The prominent dinner party had thirteen guests. One guest pointed out that a table of thirteen meant bad luck for the first one to rise. Jane was the first to get up to answer a telephone call.
In the meantime, Carlotta Adams, who liked to do impersonations of Jane, is found dead from an overdose of Veronal. The butler, along with some money, disappeared on the day of Lord Edgware's death. Ross is suspicious of Jane when he discovers she demonstrates an unusual amount of knowledge of Paris of Troy. He phones Poirot to tell him of his suspicions and while on the phone, he is murdered.
Poirot tries to piece together the three murders. He believes that Carlotta Adams impersonated Jane at the dinner party enabling Jane to take a taxi to Regent Park and murder her husband. Carlotta and Jane meet up later where Jane has promised her money for her impersonation, but gives her a fatal dose of Veronal instead. Jane then planted a case with Verona in it on Carlotta's person to make it seem like she was addicted to the drug. Jane, knowing that Ross was a risk as he pursued Poirot, murdered him. Jane's motive for killing her husband was that she wanted to marry the Duke of Merton. The Duke was a Roman Catholic who was permitted to marry a widow but not a divorcee.
Lord Edgware Dies circa 1933 courtesy http://www.agathachristie.com/stories/lord-edgware-dies.