Language learners with high working memory capacity have an advantage, all other factors being equal, during the second language acquisition (SLA) process; therefore, identifying a pedagogical intervention that can compensate for low working memory capacity would be advantageous to language learners and instructors. Extensive research on the provision of planning time before non-native speakers complete various oral and written tasks in the target language suggests that planning time might provide the processing support needed to overcome the constraints of low working memory capacity. This article reports on an empirical aptitude-treatment interaction (ATI) study of the interaction of working memory capacity and pre-task planning time with a population of English as a second language (ESL) learners in the USA. Learners were asked to complete two different oral, monologic, picture-guided narratives in two conditions: with pre-task planning time and without pre-task planning time. The results did not show an effect for the interaction of working memory capacity with pre-task planning time; that is, the results demonstrate the facilitative effect of planning time on learner fluency for participants with both high and low working memory capacity. In addition, the results demonstrate that learners with pre-task planning time produce more fluent and more complex speech than learners without pre-task planning time, with mixed results for accuracy. Finally, the results suggest that planning time may have a carry-over effect and benefit the fluency and complexity of learners’ speech during subsequent, unplanned tasks.