The Cambridge Plot (1415) was an attempt by a group of conspirators led by Richard, Earl of Cambridge, to kill Henry V and place the Earl of March on the throne. Other major conspirators included Sir Thomas Grey and Baron Scrope.

The Earl of March (Edmund Mortimer) had been the heir presumptive of Richard II. In 1399 Richard was forced to abdicate in favour of Henry IV, and for the next few decades, Mortimer served as a focal point for conspiracies aimed at removing Henry and his heirs from the throne. In 1405 Henry Percy, Earl of Northumberland, tried unsuccessfully to oust Henry IV and place Mortimer on the throne. In 1415 the Earl of Cambridge and his friends were ready to try again.

The conspirators planned to kill the king and his three brothers as they boarded a ship to France at Southampton. The plot was given away when the Earl of March himself revealed the plot to the king. All three major conspirators were taken prisoner and executed for treason at Bargate in Southampton, and their heads were impaled on spikes and kept on public display.

Henry sailed as planned to France, and eventual triumph at the Battle of Agincourt.

Henry V wisely - as it turned out - left the lesser conspirators unharmed, and as a result, faced no further challenges to the throne during the rest of his brief reign. As for Edmund Mortimer, he remained loyal to the crown. He fought with Henry in France, and in 1423 he was named lieutenant of Ireland. He died in 1425.