Jocrassa posed as Joseph Green, after killing the previous Prime Minister of Britain, as well as the real Joseph Green, becoming Acting Prime Minister. General Asquith, disappointed with his poor leadership, tried relieving "Joseph" of command under the Emergency Protocols. He and his family members killed Asquith, one of the family using his skin as a skin suit. When a group of alien experts from UNIT were gathered together in 10 Downing Street following a faked crash of the Slitheen's own ship, Jocrassa Fel-Fotch pressed a button electrocuting their ID badges, and rendering the emergency protocols in dealing with extraterrestrial encounters useless. All except the Ninth Doctor were killed.
When the Doctor was unable to convince the police that the Acting Prime Minister was an alien, Jocrassa Fel-Fotch ordered the police inside to shoot the Doctor on sight and placed the upper floors on quarantine. After the Doctor was trapped in the Cabinet Room, Jocrassa Fel-Fotch told the Earth's media that there was an alien mothership capable of deploying "massive weapons of destruction" immediately above the United Kingdom and Earth needed to prepare itself for war. He pleaded the United Nations to give the launch codes for the UK's nuclear arsenel, which were actually to be used as part of the plan to destroy the Earth.
Before the launch codes were granted to the Slitheen, Captain Price warned the Slitheen of an incoming missile. Unable to get back into his skin suit of the Prime Minister in time, Jocrassa Fel-Fotch was killed when the missile hit, along with the rest of his family (excluding Blon, who would return in the episode "Boom Town").