The goal of the proposed research project is to assess the safety (dose limiting toxicity, DLT) of the combination of pembrolizumab and high-dose stereotactic body radiotherapy (SBRT) in patients with metastatic urothelial cancer. Both the SBRT dose and pembrolizumab dose will be fixed, but the timing of the combination will be varied. Secondary objectives include response rates, local control, progression-free survival (PFS) and overall survival (OS). Exploratory endpoints include immunologic responses and response rates in PD-L1- TIL- tumors. The combination sequence with the most promising response rates and the best safety profile will be selected to continue in a Phase II trial.