In recent decades global concern about microplastics has grown considerably. Marine pollution by plastic litter has become a major global environmental issue because it affects, through the food chain, the wildlife, the natural habitats, and the human health. The project “The microplastic detectives” aimed to raise awareness of the adverse effects of the widespread presence of microplastics in the oceans and in the food chain. Through the interdisciplinary project of the STEAM methodology “The microplastic detectives” and the inquiry-based learning educational approach, students gained a basic understanding of the plastic life cycle, the term “plastic soup” on the ocean surface, the fragmentation of plastic to microplastics, and the variety of sources that plastic particles can be originated from. Moreover, they became citizen scientists by participating in meaningful scientific research about microplastics (microbeads) in cosmetic products. In addition, they did experiments to discover hidden microplastics not only in the chemistry lab but also as an outdoor activity to the water of a nearby lake. This article presents the educational goals and the contemporary teaching approaches that were followed, the web tools that were used and the expected educational results of the project.