A member of the elite Senate Guard commandos, Captain Argyus was tasked with overseeing the transport of an important Republic prisoner: Separatist mastermind Nute Gunray. Gunray's wealth and the careful manipulation of Darth Sidious ensured that Argyus' loyalties wandered. Argyus betrayed the Republic, and ensured Gunray's escape from the cruiser Tranquility.
After his arrest on Rodia, Nute Gunray was placed on the Tranquility, a Jedi Cruiser under the guard of Captain Argyus and his squadron of Commandos. In time, Argyus revealed himself to be in the employ of the Separatists and he released Gunray from his cell, allowing his escape.
Gree was unable to stop Argyus from springing Gunray from a Republic prison cell.
Luminara entrusted the security of Republic prisoner-of-war Nute Gunray to Captain Argyus, a move she would come to deeply regret.
Ahsoka Tano never suspected Argyus' true allegiance, and left prisoner Nute Gunray alone under his supervision... exactly as Argyus had planned.
Asajj Ventress' infiltration and sabotage of the Tranquility gave Argyus the cover he needed to liberate Nute Gunray. Though Argyus was expecting a sizable sum from the Separatists, he received instead Asajj's lightsaber blade.