Inclusion Criteria:

          1. At least 18 years of age. Both men and women and members of all races and ethnic
             groups will be included.

          2. Willing and able to provide written informed consent

          3. Pathologic diagnosis of rectal adenocarcinoma

          4. Stage III or Stage II with at least 1 of the following high-risk features:

               -  Distal (<1cm from anal ring)

               -  cT4 or within 3mm of MR fascia

               -  Not candidate for sphincter preservation

               -  Extramural venous invasion

          5. No prior treatment for rectal adenocarcinoma

          6. Eastern Cooperative Group (ECOG) performance status of 0-1.

          7. Laboratory values supporting acceptable organ and marrow function within 21 days of
             eligibility confirmation. Defined as follows:

               -  WBC ≥ 3,000/mL;

               -  ANC WBC ≥ 1,500/mL;

               -  PLT ≥ 100,000/mL;

               -  T Bili ≤ 1.5 x upper limit of normal (ULN);

               -  AST/ALT ≤ 2.5 x ULN;

               -  Creatinine not above ULN, or creatinine clearance ≥60 mL/min/1.73 m2 for
                  participants with creatinine levels above institutional normal.

          8. Female participants of childbearing potential (FOCBP) must have a negative serum or
             urine pregnancy test (per institutional standards) within 72 hours prior to the start
             of study drug.

             FOCBP must agree to use highly-effective method(s) of contraception (Appendix A)
             during the study and for 90 days after the last dose of study drugs.

             FOCBP are those who have not been surgically sterilized or have not been free from
             menses for >1 year without an alternative medical cause.

          9. Male participants must agree to use an adequate method of contraception (Appendix A)
             starting with the first dose of study therapy through 90 days after the last dose of
             study drugs.

        Exclusion Criteria:

          1. Distant nodal disease (retroperitoneal nodes) including inguinal nodes, or any
             metastatic disease by CT or PET

          2. Prior RT to the pelvis.

          3. Uncontrolled comorbid illness or condition including an active infection, congestive
             heart failure, unstable angina, cardiac arrhythmia, or psychiatric illness that would
             limit compliance with the study requirements.

          4. Prior treatment with any anti-PD-1, anti-PD-L1, anti-PD-L2, anti-CD137, or anti-CTLA-4
             antibody, or any other antibody or drug specifically targeting T-cell co-stimulation
             or checkpoint pathways.

          5. Any positive history for HIV/AIDS, HTLV, hepatitis B or hepatitis C virus indicating
             acute or chronic infection.

          6. Any active known or suspected autoimmune disease. Participants with vitiligo, type I
             diabetes mellitus, residual hypothyroidism due to autoimmune condition only requiring
             hormone replacement, psoriasis not requiring systemic treatment, or conditions not
             expected to recur in the absence of an external trigger are permitted to enroll.

          7. Any condition requiring systemic treatment with either corticosteroids (> 10 mg daily
             prednisone equivalent) or other immunosuppressive medications within 14 days prior to
             the first dose of study drug. Inhaled steroids and adrenal replacement steroid doses
             up to 10 mg daily prednisone equivalent are permitted (although not encouraged) in the
             absence of active autoimmune disease.

          8. Malignancy in the past 3 years that required active treatment except locally curable
             cancers or cancers deemed by the treating physicians to not impact the subject's
             survival duration.

          9. Participants receiving any other investigational agent, standard antineoplastic
             agents, or immunosuppressive agents.

         10. Known history of interstitial lung disease.

         11. Received live vaccine within 6 weeks prior to randomization.

         12. Psychiatric illness/social situations that would limit consenting and compliance with
             study requirements.

         13. Participants who are pregnant or nursing due to the potential for congenital
             abnormalities and the potential of this regimen to harm nursing infants.

         14. Patient is not a candidate for the full treatment regimen.