The field of STEM education calls for a nuanced understanding of participation as participation measured by attendance provides limited information about student learning. This multiple case study contributes to a nuanced understanding of youth's participation trajectories in a multimodal composition project. In the project, fifth to eighth grade students worked in small groups to create multimodal science fiction stories in which they needed to propose creative solutions to issues related to climate change. In this study, I adopted two theoretical perspectives, disciplinary identity development and community of practice, to analyze participation trajectories with multiple sources of data. This study shows that STEM practices mediated by multiple modes can not only offer students flexibility in moving across forms of participation, but also open space for them to demonstrate their expertise as knowledge producers. Furthermore, this study suggests that the following strategies could be effective for broadening participation in STEM practices: supporting the development of reflective understanding of connections between disciplines through digital literacies, providing exposure in composing with multiple modes, focusing on building a close relation between self and digital artifacts, and offering flexibility in moving across interactional spaces. These insights shed light on broadening participation in other multimodal learning settings.