We connected theory and research on narrative persuasion to the literature on social robots and examined the effects of a robotic storyteller's facial expressions. The congruency between the emotional arc of a story and the facial expressions of the robot was of particular interest. In a lab experiment, participants were confronted with a storytelling robot who displayed emotions that were congruent or incongruent with the events unfolding in the story or it displayed no emotions. Affect congruency increased participants' transportation in the story world, led to more positive evaluations of the robot, and increased the likelihood of choosing a product that was advertised in the story. Although the robot's voice was held constant in all three conditions, congruent facial expressions led to the illusion of a more congruent intonation by the robot. •A robotic storyteller's facial expressions were manipulated.•Story-congruent expressions were compared to story-incongruent or no expressions.•Congruent expressions yielded more transportation and story-consistent behavior.•An illusory intonation effect was observed.