Immune checkpoint inhibitors are breakthrough monoclonal antibodies in cancer therapy developed against mechanisms that suppress the immune response. After the devastating effects of chemotherapy, these specific agents have given hope to cancer patients. However, every drug has side effects itself and these useful drugs have theirs too. In addition to systemic side effects, there are also neurological side effects, the frequency of which is increasing day by day, although they are reported very rarely for now. Here, we present a case that has myositis-myocarditis-myasthenia gravis overlap syndrome. These three syndromes are very rare even to be seen alone, which are detected together. This syndrome with a very high mortality was brought under control in this case, and the fact that nivolumab treatment can be continued makes the case even more interesting. In this article, it is aimed to draw attention to this serious triple complication of immune checkpoint inhibitors and to review the relevant literature on a case basis.