Plastic was initially designed to enhance human well-being, but now it has become a threat to us and the planet. The plastic industry faces significant scrutiny due to the sustainability challenges associated with plastic products. The purpose of this study is to understand how organisations in the plastic industry manage its legitimacy and ensure the industry's survival. We employ organisational legitimacy as a lens and the three pillars of institutions - regulative, normative, and cultural-cognitive - as an analytic framework, to understand the roles of legitimacy in shaping an organisation's sustainability practices. Site Zero, Europe's largest plastic packaging waste sorting facility in Motala, Sweden has caught our attention. Utilising a qualitative research strategy and a case study research design, this paper explores how the multifaceted nature of legitimacy impacts Site Zero on its sustainability practices. Besides the site visit, we have conducted seven semi-structured interviews with five key members of Swedish Plastic Recycling and two material experts from a public agency and a producer responsibility organisation. Through this case study, we conclude that there is a gap between legitimacy and sustainability. Pursuing legitimacy increases the chance of organisations' survival in the plastic industry, however, it may also hinder them from achieving sustainability. Additionally, while high-quality recycling plays a crucial role in handling plastic packaging waste, it has its limitations in making plastic sustainable with the current profit-driven market and an overconsumption lifestyle.