In the mid-1980s, researchers began to study writers working in collaboration. Much of this research attended to what might be termed side-by-side composing: authors working on their own individual pieces and discussing them with others as needed. Others have studied co-composing--that is, multiple authors crafting a single text--describing the various aspects of these collaborations and their constraints and affordances. Nevertheless, few of these researchers have examined settings in which children spontaneously undertake co-composing. The study described here is a case analysis of a group of third-grade boys who chose to work together to write a superhero story over a period of six weeks. Mobilizing Bakhtin's construct of heteroglossic discourse, the study explores interactions among the authors and aspects of their final product. Classroom observations, audio-recordings of co-composing sessions and student interviews, as well as the finished text served as the triangulated data for the study. Analysis showed that both the students' interaction and the text that resulted from it was multi-voiced in nature. I argue here for a Writing Workshop model that foregrounds student choice and agency, rather than asserting that procedures are the primary drivers of success. In the context described here, such a model allowed students in this classroom to follow their own interests and work with peers who were supportive of those interests.