Children using robotics for engineering, science, technology, and mathematics (CREST-M) is an ongoing curriculum creation and evaluation project developing math-focused science, technology, engineering, and mathematics (STEM) curriculum units that incorporate storytelling with math and are designed to be engaging for students from backgrounds traditionally underrepresented in STEM fields and to be appropriate for all ability levels including the gifted. This mixed-methods study evaluated one CREST-M unit, "The Tale of the Monarchs," aligned to the Common Core State Standards: Mathematics (CCSS-M) for fourth- and fifth-grade fractions while incorporating all aspects of STEM. The unit includes an engineering design loop to help students create and program robots using LEGO WeDo 2.0. Students also use the scientific method in a problem-based learning (PBL) investigation in which they design and conduct experiments. The unit features a comprehensive storyline in which diverse characters solve real-world problems using STEM skills. The curriculum was evaluated through a CCSS-M aligned pre- and post-assessment of students' understanding of fractions, the Cognitive Abilities Test Screening Form 7 (CogAT 7), and the Draw-a-Scientist pre- and post-assessment. The curriculum led to significant and meaningful gains in math achievement (Cohen's d = 0.72) consistent for students at multiple ability levels (including high ability) and children from groups traditionally represented in STEM fields as well as those traditionally underrepresented (across gender, race, and socioeconomic status). In addition, the mental models of the career of a scientist that students articulated before the program were markedly more complex and less stereotypical following the program, indicating increased understanding of the career of a scientist.