As multimodal composition is gradually integrated into STEM learning, research is needed to examine how to fully connect multimodal composing practices and STEM practices to support students' STEM identity exploration. To fill in this gap, we conducted a design-based research project to investigate how students explored STEM identities in a multimodal composing environment. A total of 42 fifth- to eighth-grade students participated in the project in which they worked in groups to create multimodal science fiction stories. We identified two cross-cutting themes regarding how students presented STEM-related selves in multimodal artifacts: channeling current and imagined future life experiences into multiple characters and experiencing a hypothetical universe through restorying the self. The findings suggest that students presented a mix of current and future selves in multimodal artifacts. This study sheds light on promoting STEM identity by engaging students in presenting selves through modes of choice.