The purpose of the study is to determine whether an investigational drug called pembrolizumab combined with standard-of-care stereotactic radiation therapy and surgery is safe and effective in patients with recurrent glioblastoma. Researchers will evaluate whether the drug improves overall survival, increases the length of time during and after treatment that the patient’s cancer does not worsen, and how well the body responds to treatment. Participants will receive one dose of pembrolizumab intravenously before their planned radiation therapy and surgery. After surgery, they will receive study drug infusions every 6 weeks until disease progression, intolerable side effects, withdrawal of consent, or up to 2 years or 18 cycles of pembrolizumab, whichever comes first. Blood and tumor samples will be collected to evaluate biomarkers. A biomarker is a biological molecule found in blood, other body fluids, or tissues that may be a sign of a condition or disease.