The ecotoxicity of large plastics resulting from adsorbed chemicals is largely unexplored. In this study, through an ecotoxicological simulation, we demonstrate how large plastics act as vectors of micro-organic pollutants adsorbed by them. Green mussels Perna viridis were exposed to low-density polypropylene (LDPE) plastics and LDPE plastics treated with highly toxic and carcinogenic PAH benzo[a]pyrene (BaP) in multiple tanks over a period of 21 days. Mussel tissues, gills, and intestines of mussels were harvested on day 0, 7, 14, and 21 to measure BaP levels in tissues, total protein concentrations, and antioxidant biomarker (SOD, CAT, and GPx) activities in gills and intestines. The results showed that BaP uptake by mussels took place after one week and peaked on the 14th and 21st days. Survival probability decreased from 80% on day 7 to 20% on day 14 in the LDPE + BaP group. SOD levels in the intestine were much lower than those in the gills, indicating that the toxicity of BaP to Perna viridis was more severe in the gills than in the intestine. This was further confirmed by the high negative correlation (R2 = 0.79) between BaP concentration and SOD activity levels. In the LDPE + BaP exposure tank, CAT activity increased consistently in gills from day 7 to day 14 to day 21, while for intestines, the levels increased from day 7 to day 14, then dropped on day 21. A positive correlation between CAT levels and BaP concentrations was noted for both gills and intestines (R2 = 0.86). In gills, very low levels of increasing CAT may not have a distinct influence on the overall detoxification process in P. viridis. The decreasing activity of GPx was possibly because both CAT and GPx acted on the same substrate, while CAT was the first responder to reduce peroxides to water, as evident from the multi-correlation analysis showing a strong negative correlation between CAT and GPx. Overall, our study could have important implications in the biomonitoring of marine mussels exposed to plastic-attached toxic pollutants.