Immune checkpoint inhibitors (ICIs) block inhibitory pathways that tumour cells exploit to evade the immune system. They play a key role in the management of aggressive cancers, including non-small cell lung cancer, melanoma and Hodgkin's lymphoma. Despite their efficacy, ICIs can cause severe, potentially fatal, immune-related adverse events (irAEs), including multiorgan failure. We present a case of a woman in her early 70s with metastatic lung adenocarcinoma recently initiated on pembrolizumab who presented with an acute stroke complicated by irAE-related multiorgan failure. Despite aggressive treatment, the patient remained critically ill and passed away the same day after transitioning to end-of-life care. This case emphasises the life-threatening potential of irAEs, the importance of early diagnosis and the challenges of managing these complications. Clinicians should maintain a high index of suspicion for irAEs in ICI-treated patients to optimise timely intervention and outcomes.