Inclusion Criteria:

          -  Age 18 years or older

          -  ECOG performance status 0-1

          -  Endometrial cancer: all histologies except for carcinosarcoma

          -  MMR-D or MSI-H by IHC, PCR or NGS testing. For patients with outside testing, archival
             tissue will be required for NGS testing at MSKCC

               -  Patients with hypermethylation of MMR gene promoters (MLH1, MSH2, MSH6 or PMS2)
                  will be included

               -  POLE-mutated endometrial cancers will be included

          -  Must have undergone a complete surgical staging and have stage III/IVA disease.

               -  Patients with residual disease after surgery can still be enrolled with the
                  exception of peritoneal implants. Those with residual disease after surgery
                  should be reviewed and approved by the study PI.

               -  PatienSurgery completed between 3 weeks and 12 weeks (inclusive) before cycle 1
                  day 1 of therapy, and must have adequately recovered from surgery and any
                  complications of surgery.ts with isolated tumor cells in lymph nodes found on
                  surgery will be counted as clinical stage III disease

          -  Surgery completed between 3 weeks and 12 weeks (inclusive) before cycle 1 day 1 of
             therapy, and must have adequately recovered from surgery and any complications of
             surgery.

          -  Has a negative serum pregnancy test within 7 days prior to taking study treatment if
             of childbearing potential, and agrees use an adequate method of contraception from
             screening through 150 days after the last dose of study treatment, or is of
             nonchildbearing potential. Nonchildbearing potential is defined as follows (by other
             than medical reasons):

             °≥45 years of age and has not had menses for >1 year

               -  Patients who have been amenorrhoeic for <2 years without history of a
                  hysterectomy and oophorectomy must have a follicle stimulating hormone value in
                  the postmenopausal range upon screening evaluation

               -  Post-hysterectomy, post-bilateral oophorectomy, or post-tubal ligation.
                  Documented hysterectomy or oophorectomy must be confirmed with medical records of
                  the actual procedure or confirmed by an ultrasound. Tubal ligation must be
                  confirmed with medical records of the actual procedure, otherwise the patient
                  must be willing to use an adequate birth control method throughout the study,
                  starting with the screening visit through 150 days after the last dose of study
                  treatment. See list of acceptable birth control methods. Information must be
                  captured appropriately within the site's source documents. Note: Abstinence is
                  acceptable if this is the established and preferred contraception for the
                  patient.

          -  Participant must agree to not breastfeed during the study or for 150 days after the
             last dose of study treatment.

          -  Demonstrate adequate organ function as defined below. All screening labs should be
             performed within 14 days of treatment initiation.

               -  Absolute neutrophil count (ANC) ≥1,500 /mcL

               -  Platelets ≥100,000 / mcL

               -  Hemoglobin ≥9 g/dL

               -  Serum creatinine GFR ≥ 45

               -  Serum total bilirubin ≤ 1.5 X ULN OR Direct bilirubin ≤ ULN for subjects with
                  total bilirubin levels > 1.5 ULN. Except patients with Gilbert's disease (≤3x
                  ULN)

               -  AST and ALT ≤2.5 ULN OR ≤ 5 X ULN for subjects with liver metastases

               -  Albumin >3 mg/dL

               -  International Normalized Ratio (INR) or prothrombin time (PT) ≤1.5× ULN unless
                  patient is receiving anticoagulant therapy as long as PT or partial
                  thromboplastin (PTT) is within therapeutic range of intended use of
                  anticoagulants

               -  Activated partial thromboplastin time (aPTT) ≤1.5× ULN unless patient is
                  receiving anticoagulant therapy as long as PT or partial thromboplastin (PTT) is
                  within therapeutic range of intended use of anticoagulants

        Exclusion Criteria:

          -  Has had prior chemotherapy, targeted therapy, or investigational therapy for
             endometrial cancer.

             °Patients who have received prior chemotherapy, targeted therapy, or other
             investigational therapies for other cancers and are in remission/cured may be included
             at the discretion of the treating investigator in consultation with the study PI.

          -  Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti-CTLA-4 agent for
             endometrial cancer.

          -  Carcinosarcoma histology

          -  Unfit for Radiation Therapy due to the following:

               -  Has had radiation therapy encompassing >20% of the bone marrow within 2 weeks; or
                  any radiation therapy within 1 week prior to Day 1 of protocol therapy.

               -  Patients with a prior history of pelvic radiation

               -  Patients with a prior known history or current diagnosis of a vesicovaginal,
                  enterovaginal, or colovaginal fistula.

               -  Any hematological abnormality or disorder that would be a contraindication to
                  radiation per the treating physician.

          -  Hypersensitivity to TSR-042 or any of its excipients

          -  Patients with diagnosis of immunodeficiency or patients receiving systemic steroid
             therapy or any other form of immunosuppressive therapy within 7 days prior to the
             first dose of trial treatment.

          -  Patients who have received acute, low dose, systemic immunosuppressant medications
             (e.g., dexamethasone containing antiemetic regimen or steroids as CT scan contrast
             premedication) may be enrolled.

               -  The use of inhaled corticosteroids and mineralocorticoids (e.g., fludrocortisone)
                  for patients with orthostatic hypotension or adrenocortical insufficiency is
                  allowed.

               -  Total systemic steroid dose cannot exceed an equivalent of 10mg of prednisone
                  daily and dose must be stable for at least 4 weeks prior to initiating protocol
                  therapy

          -  Has active TB (Bacillus tuberculosis)

          -  Has evidence of active, non-infectious pneumonitis.

          -  Has history of interstitial lung disease

          -  Has an active infection requiring systemic therapy with intravenous antibiotics

          -  Has a history or current evidence of any condition, therapy, or laboratory abnormality
             that might confound the results of the trial, interfere with the subject's
             participation for the full duration of the trial, or is not in the best interest of
             the subject to participate, in the opinion of the treating (within 90 days) myocardial
             infarction, chronic obstructive pulmonary disease, uncontrolled major seizure
             disorder, unstable spinal cord compression, and superior vena cava syndrome.

          -  Has psychiatric or substance abuse disorders that would interfere with cooperation
             with the requirements of the trial.

          -  Prior malignancy within the past 2 years prior to start of treatment, with the
             exception of early stage cancers with low likelihood of recurrence in the opinion of
             treating investigator.

          -  Patients with prior allogeneic bone marrow transplantation or prior solid organ
             transplantation.

          -  Active or prior documented autoimmune or inflammatory disorders (including
             inflammatory bowel disease; systemic lupus erythematosus; Wegener syndrome
             [granulomatosis with polyangiitis]; myasthenia gravis; Graves' disease; rheumatoid
             arthritis, hypophysitis, uveitis) within the past 3 years prior to the start of
             treatment or patients requiring immunosuppressive therapy for the autoimmune disease.
             The following are exceptions to this criterion:

               -  Subjects with vitiligo or alopecia

               -  Subjects with hypothyroidism (e.g., following Hashimoto syndrome) stable on
                  hormone replacement or psoriasis not requiring systemic treatment.

          -  Has a history of uncontrolled Human Immunodeficiency Virus (HIV) (HIV 1/2 antibodies).

             °Patients with well-controlled HIV on therapy with undetectable viral loads and
             absolute CD4 count above 300 may be included.

          -  Has active Hepatitis B (e.g., HBsAg reactive/HBV DNA PCR positive) or Hepatitis C
             (e.g., HCV RNA [qualitative] is detected).

             °Patients with a positive Hep B cAb will require prophylactic therapy prior to
             enrollment.

          -  Has received a live vaccine within 30 days of planned start of study therapy.

             °Note: Seasonal influenza vaccines for injection are generally inactivated flu
             vaccines and are allowed; however intranasal influenza vaccines (e.g., Flu-Mist®) are
             live attenuated vaccines, and are not allowed.

          -  Is unwilling to give written informed consent, unwillingness to participate, or
             inability to comply with the protocol for the duration of the study. The use of LAR
             for cognitively impaired patients is not permitted.