Geometry is a branch of mathematics that helps in developing critical thinking, deductive reasoning, and problem solving through a comprehensive understanding of mathematical concepts. In contrast to this importance, secondary school students faced significant challenges in understanding plane geometry concepts. As a result, their lack of knowledge made it more difficult for them to understand basic ideas and to find solutions to everyday problems. Thus, the study examined the impact of the Van Hiele group-guided discovery instructional approach on tenth-grade students' motivation, engagement, conceptual understanding, and problem-solving abilities using a quasi-experimental, non-equivalent control group design. Further, the study answered whether the groups made statistically significant differences between them before and after the intervention in each dependent variable. In addition, the study shows the contribution of student motivation, engagement, and conceptual understanding on problem-solving abilities. Three carefully selected secondary schools in Hawassa, Ethiopia, took part in the main study focused on tenth-grade students. The control group was assigned to the school that was slightly farther away from the other two, while Experimental Group I and Experimental Group II were randomly assigned to the other two schools. To gather data, the study employed focus group discussion with selected tenth grade students, interviews with the mathematics teachers of those students, observation of the researcher, and pre-post-tests. ANOVA, Paired sample t-test, ANCOVA, and thematic analysis were used to analyze the data. Its success above the conventional way was demonstrated by the study, which found that the van Hiele group-guided discovery instructional approach greatly improved students' learning. The study concluded that the Van Hiele group guided-discovery instructional approach improves students' understanding of plane geometry and suggested that secondary school mathematics teachers use this integrated approach when teaching plane geometry. The study recommended that researchers test the effect of this teaching strategy's effectiveness at the primary school, higher education institutions, and in other subjects.