A sequential mixed-methods action research study was undertaken with a group of 10th-grade students enrolled in a required English course at an independent secondary school. The purpose of the study was to investigate students' negotiation of agentic writer identity in a course that featured a three-strand intervention: (a) a high degree of student choice; (b) ongoing written self-reflection; and (c) ongoing instruction in mindset. The researcher drew on self-determination theory and identity theory to operationalize agentic writer identity around three constructs—behaviors, identity, and belief. A questionnaire was used to identify an array of cases that would illustrate a range of experiences around agentic writer identity. Questionnaire data were analyzed to identify a sample from which to collect qualitative data and to identify prominent central relations among the three constructs, which were further explored in the second stage through the qualitative data. Qualitative data were gathered from a primary group of six students in the form of student journals and interviews around the central constructs of writing belief, writing behavior, and writer identity. Using a snowballing sampling method, four students were added to the sample group to form a second tier of data. The corpus of qualitative data from all 10 students was coded and analyzed using the technique of re-storying to produce a narrative interpretation, in the style of the Norse saga, of students' engagement in agentic writing behaviors, espousal of agentic writing beliefs, and construction of agentic writer identities. A defense of the chosen narrative approach and genre was provided. Interpretation of the re-storied data was provided, including discussion of interaction among themes that emerged from the data and the re-storying process. Emergent themes and phenomena from the re-storied data were realigned with the quantitative data as well as with the constructs that informed the survey design and sampling. Implications for classroom teachers, as well as suggestions for further research, were suggested.