Our thesis in mathematics education explores the relevance of story building as a pattern of thought and potential support for the construction of reasoning. We assume that establishing a narrative in order to answer a problematic situation is both a trigger and processing aid. Our main hypothesis is that the narrative act offers structured guidelines for the process of reasoning during problem solving activity. Several studies have indeed highlighted the heuristics and structuring functions of narrative. Relying on these studies we first characterized theoretically and then tested experimentally a didactical environment. We shaped the characteristics of this environment with the objective of ensuring a joint development between narrative and reasoning. The situation of problem solving we offer is built around a game. To address problematic situations, students produced descriptive (based on games actually made) and anticipated (based on imaginary games) narratives. In both case, the construction of a narrative is a powerful asset. Our results demonstrate that the story enriches, within the meaning of Hersant (2010), the educational environment. The act of narration supports the student’s mathematical reasoning and justification. While producing different types of stories, students embrace the mathematical constraints of the situations and step back from the sensible world. No more restrained by material thinking, the students are more inclined to mathematical approach. Therefore, they develop or enhance their mathematical proving skills (example and counter-example arguments, mathematical conjectures and demonstrations, etc.)