Neo-adjuvant Pembrolizumab in Primary Stage IV Ovarian Cancer

Inclusion Criteria: * Written informed consent for the trial. * Diagnosis of primary stage IV high-grade serous ovarian, peritoneal, or fallopian tube cancer. * Age \>= 18 years on day of signing informed consent. * Willing and able to provide three tumor biopsies (1 FFPE, 2 fresh frozen) prior to start of treatment * Performance status of 0 or 1 on the ECOG Performance Scale. * Adequate organ function as defined in Table 1 of the protocol * Female subject of childbearing potential should have a negative urine or serum pregnancy within 72 hours prior to receiving the first dose of study medication. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required. Exclusion Criteria: * Previously received treatment for ovarian, peritoneal, or fallopian tube cancer. * Has had a prior anti-cancer monoclonal antibody (mAb) within 4 weeks prior to study Day 1 or who has not recovered (i.e., ≤ Grade 1 or at baseline) from adverse events due to agents administered more than 4 weeks earlier. * Has had prior chemotherapy, targeted small molecule therapy, or radiation therapy within 2 weeks prior to study Day 1 or who has not recovered (i.e., ≤ Grade 1 or at baseline) from adverse events due to a previously administered agent. * Known additional malignancy, unless treated with curative intent without chemotherapy at least five years ago. In situ cancers, basal cell carcinoma of the skin or squamous cell carcinoma of the skin that have undergone potentially curative therapy within the past five years may also be eligible. * Currently participating and receiving study therapy or has participated in a study of an investigational agent and received study therapy or used an investigational device within 4 weeks of the first dose of treatment. * A diagnosis of immunodeficiency or is receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of trial treatment. * A known history of active TB (Bacillus Tuberculosis) * Hypersensitivity to pembrolizumab or any of its excipients. * Has had a prior anti-cancer monoclonal antibody (mAb) within 4 weeks prior to study