Southampton Plot

The Southampton Plot of 1415 was a conspiracy to replace King Henry V with Edmund Mortimer, 5th Earl of March.

Mortimer was the great-grandson of Lionel of Antwerp, 1st Duke of Clarence, second surviving son of King Edward III, and his claim to the throne was thus superior to that of Henry V and his father, Henry IV, who derived their claim from Henry IV's father, John of Gaunt, 1st Duke of Lancaster, third surviving son of Edward III. Moreover, Edmund Mortimer's father, Roger Mortimer, 4th Earl of March, had been widely considered heir presumptive to King Richard II, who had no issue, and Edmund Mortimer himself had been heir presumptive to Richard II while a young child.

The three ringleaders of the plot were Edmund Mortimer's brother-in-law, Richard of Conisburgh, 3rd Earl of Cambridge; Henry Scrope, 3rd Baron Scrope of Masham (whose uncle Richard le Scrope had been executed for his part in a 1405 revolt also supporting Mortimer's right); and Sir Thomas Grey, whose son, Thomas, had been betrothed in 1412 to Cambridge's only daughter, Isabel.

The nominal principal, the Earl of March, informed King Henry of the plot on 31 July, stating that he had only just become aware of it. Richard, Scrope, and Grey were promptly arrested. The trial took place in Southampton, on the site now occupied by the Red Lion Inn. Grey was beheaded on 2 August and the two peers on 5 August, both in front of the Bargate. Satisfied, Henry sailed for France on 11 August.

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