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The 'Southampton Plot' of 1415 was a conspiracy against Henry V of England, aimed at replacing him with Edmund Mortimer, 5th Earl of March. The three ringleaders were Richard of Conisburgh, 3rd Earl of Cambridge, Mortimer's brother-in-law, Henry Scrope, 3rd Baron Scrope of Masham (whose uncle Richard le Scrope had been executed for his part in the 1405 revolt), and Sir Thomas Grey of Heton. They were charged with plotting to murder Henry at Southampton before his embarkation into France; revolts in favour of Mortimer by Lollards under Sir John Oldcastle in the West Country, and by the Percies in the North, would follow.
The plot was disclosed to its nominal principal, the Earl of March, shortly before it was to take effect. However, he promptly informed King Henry (on July 31), and the three ringleaders were promptly arrested and put to death (Grey on August 3 and the two peers on August 5). Satisfied, Henry sailed for France on August 11.