Integrated writing has gained importance in both tertiary and secondary learning contexts. However, assessment tasks at secondary schools have received little attention. To obtain evidence of skill integration, a task-defining feature for integrated writing, we developed a comprehensive questionnaire study of an understudied task used in the secondary EFL context, namely, the story continuation writing task. An exploratory factor analysis of 258 secondary students' responses to a self-generated questionnaire and a confirmatory factor analysis of another 470 responses to a revised questionnaire revealed a correlated structure of planning, writing, grammar, and a secondary-order factor reading-to-connect comprising "reading and selecting" and "transformation". The structure model showed that writing affected test performance directly and positively, and grammar and reading-to-connect mediated the impact of planning on writing. The findings indicate that the story continuation writing task is a content-responsible task tapping heavily into writing. The skill integration continuum is proven to be a useful framework for understanding the integrated writing construct.