Health and safety concerns have led to policies that put individuals under lockdown, but such restrictions lose effectiveness in the longterm due to inherent human needs of connection and physical action. People maintain prosocial behaviors long-term only if they make decisions themselves intrinsically as opposed to forced restrictions. To build systems for effecting positive social purpose in pandemic and environmental concerns, we apply speculative design to create story structures and interactions that promote behaviors for social good. We designed stories and interactions using both plot-based narrative frameworks and character-based machine-learninggenerated dialogues for effecting cooperation. We then ran a series of workshops investigating how designers negotiate and collaborate to tell stories for social purpose using a "finish each other's stories" approach. This work illustrates the application of design fiction to promote sustainable behavioral patterns that value societal good.