This study is investigating a new treatment option for bladder cancer. It tests if adding a drug called **pembrolizumab** to **radiation therapy** is better than using the usual **chemotherapy** during radiation. Pembrolizumab is a type of **immunotherapy** that helps your immune system fight cancer. Chemotherapy uses strong drugs to stop cancer from growing. Radiation therapy uses high-energy rays to kill cancer cells. **Key Points:** - **Duration:** The study lasts several years with follow-up visits every 3 months for 2 years, then every 6 months for 3 years, and annually for 5 years. - **Treatments:** Patients receive either chemotherapy with radiation or pembrolizumab with radiation. - **Safety:** The study monitors the safety and quality of life for participants. Participants should be at least 18 years old, have a specific type of bladder cancer, and meet other health criteria to join. This study does not allow those with certain health conditions, previous treatments, or allergies to participate. The main goal is to see if this new combination keeps the bladder healthy longer.