Competencies for designing and evaluating scientific inquiries are a key emphasis in national curriculums worldwide and a focus of international large-scale assessment. Teachers globally are encouraged to provide learning opportunities for their students to explore and practice asking and conducting researchable questions. This study focused on the competencies of formulating researchable questions and conducting quality investigations, which are the cornerstones of the scientific method. Thirty-eight (n = 38) second year, Australian undergraduate pre-service teachers (aged 19-40 years) participated in a semester-long course of teaching intervention. The study investigated pre-service teachers' pre- and post-affective attitudes toward science, epistemic beliefs, and developmental performances in the context of formulating and planning practical science investigations. T-test results demonstrated that pre-service teachers' post- results in "formulating research questions" (t = -8.34, p < 0.001, d = 1.40) and "planning investigations" (t = -7.83, p < 0.001, d = 1.34) were significantly higher than their pre- scores. Additional structural relationship analyses revealed that in the early stages of developing a scientific investigation, the positive and significant predictive coefficient for pre-service teachers in "formulating research questions" was "epistemological beliefs." In the final reporting stage of the investigation, pre-service teachers' "scientific affective attitudes" positively predicted their "improved planning investigations." The results highlight the unique affordance of providing practical experiences of fair testing in pre-service science teacher education.