Background: Michael John Harrison (born 26 July 1945), known for publication purposes primarily as M. John Harrison, is an English author and literary critic. His work includes the Viriconium sequence of novels and short stories (1971-1984), Climbers (1989), and the Kefahuchi Tract trilogy, which consists of Light (2002), Nova Swing (2006) and Empty Space (2012). He is widely considered one of the major stylists of modern fantasy and science fiction, and a "genre contrarian". The Times Literary Supplement described him as 'a singular stylist' and the Literary Review called him 'a witty and truly imaginative writer'.
Context: Harrison's enduring fantasy sequence concerning the fictitious city of Viriconium consists of three novels and various short stories and novels written between 1971 and 1984. Viriconium is known as the Pastel City. Both universal and particular, the city has a shifting topography and history, and is sometimes known by names such as 'Uroconium'.  The first book, The Pastel City (1971), presents a civilization in decline where medieval social patterns clash with the advanced technology and superscience energy weapons that the citizens of the city know how to use but have forgotten how to engineer. The more complex second novel is A Storm of Wings (1982). It is set eighty years later than The Pastel City. and stylistically it is far denser and more elaborate. A race of intelligent insects is invading Earth as human interest in survival wanes. Harrison brilliantly depicts the workings of civilization on the verge of collapse and the heroic efforts of individuals to help it sustain itself a little longer.  The third novel, In Viriconium (1982) (US title: The Floating Gods), was nominated for the Guardian Fiction Prize during 1982. It is a moody portrait of artistic subcultures in a city beset by a mysterious plague. Where the previous books in the series held some sword and sorcery elements, In Viriconium goes beyond black humour into a coma of despair.  The short story "A Young Man's Journey to Viriconium" (1985; later retitled "A Young Man's Journey to London") is set in our world. It explains that Viriconium can be visited via a mirror in a bathroom in a cafe in England.
Question: What is the plot of the first book?. Whats the answer?
a civilization in decline where medieval social patterns clash