Theory and research have linked pretend play in early childhood with the development of language and theory of mind. In 102 mother–child dyads at 4.5 years, we examined whether (1) introducing a story stem (a play narrative with socioemotional dilemmas) in a mother–child play context increases pretend play complexity compared with mother–child free play; and (2) maternal sensitivity is associated with pretend play complexity. Further, we explored whether the story stem increased child pretend play complexity more in dyads with mothers with low sensitivity compared with highly sensitive mothers. Sensitivity was coded using Coding Interactive Behavior and pretend play complexity with a global, integrated measure of the developmental level and quantity of play. Using generalized estimating equations, we found that pretend play complexity was positively associated with introducing a story stem and maternal sensitivity. Mixed methods ancova showed no significant interaction between play situation and maternal sensitivity. The findings stress the importance of maternal sensitivity and participation for play and how introducing a story stem may help promote child pretend play complexity.