Using enhanced rust, the Doctor destroyed the ship Baltazar had built, Baltazar having destroyed the entire Earth defence. With his space piracy, cybernetics, robot parrot, and desire to crush planets into precious gems, Baltazar bears a striking resemblance to The Captain, a character from the Fourth Doctor adventure, The Pirate Planet. Baltazar destroyed a planet in the 40th century, amongst other crimes, and was sent to the prison planet Volag-Noc. With the aid of Caw, a robotic parrot, Balthazar puts a tracking device on the Doctor so the Doctor and Martha would find The Infinite – a huge spaceship that can grant people their heart's desire – for him.
Once he finds The Infinite he discovers an illusion showing "what the heart desires". After the Doctor uses his sonic screwdriver to tear the ship apart, he orders Squawk to escort him backto Volag-Noc where and imprisonment.
The title "Scourge of the Galaxy" previously belonged to the Macra.