sentences:  With only two weeks to spare before filming, Read and Williams quickly co-wrote a replacement script in the form of "The Invasion of Time".When asked about Weir's story at a fan convention years later, Williams could not recall its title and made up the name "The Killer Cats of Geng Singh", by which title the story became widely known in fan circles.Written by Moris Farhi, this four-part story was officially commissioned by producer Graham Williams on 8 November 1977.