Garce was an English spy in an alternate Paris. He and Eging captured and murdered Sophie. They used her innocent blood to contact the Masksmasters. They were told to contact the TARDIS' occupants, since they were from outside the alternate universe and might be able to help.
Later, when Larkspur and Tyll Howlglass were expelled from the Pageant and went to contact the people from the TARDIS themselves, the rest of the Pageant told Eging and Garce to kill the renegades. After doing so, Eging tried to shoot Fantômas, but Bressac jumped in the way. Driven crazy by the dying screams of Larkspur and Howlglass, Garce shot Eging in the head and ran. Fantômas pursued and killed him. (PROSE: The Man in the Velvet Mask)