Theoretically framed within Social Cognitive Career Theory emphasising on the construct of situational interest, this study explores the impact of career-based scenarios, as an instructional approach, on students' interest in science and understandings of STEM careers. This case study involved 16 students aged 13-15 years old, who participated in a classroom intervention consisting of five sessions in a period of two years. Data were collected through a questionnaire and semi-structured interviews administered repeatedly after each session. The questionnaire data were analysed with the use of descriptive statistics and the interview data were analysed through content analysis. The findings illustrate that opportunities for active engagement in scientific practices and interactions with experts were important conditions for career-based scenarios to succeed in enhancing students' situational interest and understandings of STEM careers. These findings hold important implications for educational practice as they offer insights into career-oriented curriculum design for the purpose of enhancing student interest in science as well as understandings of STEM careers.