The present study aims at investigating the effects of narrative complexity of storyline (presence of simultaneous events) on EFL learners’ oral performance, as displayed by its complexity, fluency, and accuracy. Forty-two Iranian learners of English with two different proficiency levels performed two narrative tasks (with or without background events) based on two picture stories. The performances, then, were recorded, transcribed, and coded to measure the complexity, fluency, and accuracy of the participants’ performances. Next, the matched t-test was employed to analyze the collected data. The results indicated that syntactic complexity of performance was related to the storyline complexity, i.e. for both groups, more syntactic complexity was associated with the narrative that had both foreground and background storylines. Storyline complexity also helped high-proficiency learners to perform more fluently. In neither group did storyline complexity have a significant effect on accuracy. Findings have pedagogical implications for the field of syllabus design.