Pembrolizumab, a monoclonal antibody that inhibits programmed cell death protein-1 (PD-1), is an important treatment for various malignancies. Unfortunately, it has also been associated with a wide array of immune-related adverse events. We present a unique case of a patient who received a single dose of pembrolizumab and subsequently developed multiple immune-mediated complications, including dermatitis, hepatitis, myositis, myocarditis, and myasthenia gravis.