Extant research and engineering education frameworks call for students to engage in personally meaningful engineering projects; however, there are few case studies documenting the work of young engineers working to design solutions to real-world problems that matter to them. This qualitative case study describes the work of a purposively selected group of high school engineering students (the InvenTeam) (n = 15) as they devote a school year to a particularly ambitious invention project: designing and prototyping a device to mitigate deaths occurring when children are left unattended in hot cars. Utilizing cultural historical activity theory as a theoretical and analytical lens, the study triangulates observation, interview, and document data to describe elements of and tensions within the InvenTeam activity system. Data illustrate numerous ways in which invention afforded opportunities for students to apply science, technology, engineering, and mathematics (STEM) knowledge and practices and develop engineering identities. Additionally, data suggest that students' purposes for engaging in the project were dynamic, telescoping from individual, personal aspirations to expanded possibilities for economic and societal impact. Data also illustrate how students assumed defined yet flexible roles within the project's division of labor. Activity systems analysis revealed four main tensions within the InvenTeam activity system: sustaining motivation in the face of technical challenges, community expectations versus student goals, STEM knowledge/skills constrained by specialized roles, and the institutional norms of schooling versus the process of invention. Implications of the case study findings for engineering and invention at the high school level are discussed.