Cassandra Clare’s fantasy trilogy The Mortal Instruments is headed to the big screen, Variety reports. Jessica Postigo will write the script, based on the novels City of Bones, City of Ashes and City of Glass. The story follows a teenager’s search for her missing mother, taking her to a fantastical city. Unique Features partners Bob Shaye and Michael Lynne, formerly of New Line, are working with Constantin Film on the project.