He hoped to infiltrate his agent Ninian Cockburn into Leith posing as the messenger .
Throckmorton intercepted a letter meant for Jacques de la Brosse from Mary of Guise 's brothers .
Throckmorton hoped to infiltrate Throckmorton 's agent Ninian Cockburn into Leith posing as the messenger .
Throckmorton intercepted a letter meant for Jacques de la Brosse from Mary of Guise 's brothers .