After Universal Pictures acquired the film and distribution rights from Fox, Whedon began writing the screenplay for "Serenity". Transforming the series into a film, he says, "... was the hardest piece of writing I've ever done ... It had to be self-contained and work as a movie, which meant I had to cope with problems like introducing nine main characters who'd already met!" The script was based on unused story ideas for "Firefly"s unfilmed second season. On writing the dialogue, Whedon felt that part of it came from "getting to invent the language", which "once I had... reads like a kind of poetry". The narrative centered on Captain Malcolm Reynolds as the hero accompanied by River Tam acting as the catalyst for what he does.