This study investigates the combined effects of task-based careful online planning and the storyline structure of a task on second language performance (complexity, accuracy and fluency). Sixty intermediate EFL learners were randomly assigned to four groups (n = 15). Participants were asked to perform two tasks with different degrees of storyline structure (structured and unstructured) under two different planning conditions (pressured online planning and careful online planning). Analysis of the narrations and the results of a series of one-way ANOVA revealed that the participants who performed the structured task under the careful online planning condition produced more complex, accurate and fluent language. However, those who performed the unstructured task under the pressured online planning condition obtained the lowest scores in all three areas of oral production. The findings add support to the view that selecting appropriate task-based implementation conditions and task design features can induce language learners to increase the complexity, accuracy and fluency of their output.