Inclusion Criteria:

          1. Signed Informed Consent Form for Registration

          2. Male or female patients ages 16 years or older

          3. Eastern Cooperative Oncology Group (ECOG) performance status 0, 1 or 2

          4. Histologically proven primary triple negative breast cancer as defined as oestrogen
             receptor (ER) negative, progesterone receptor (PgR) negative (if available, otherwise
             PgR unknown), (as defined by Allred score 0/8 or 2/8 or stain in <1% of cancer cells)
             and human epidermal growth factor receptor 2 (HER2) negative (immunohistochemistry
             0/1+ or negative by in situ hybridization) as determined by local laboratory.

          5. Availability of tissue from two archival tumour tissue samples (either from diagnostic
             biopsy, and/or primary surgery. If only one tumour sample is available, the site
             should inform the ICR-CTSU who will discuss eligibility with the Chief Investigator or
             the designated Trial Management Group (TMG) member. Patients who have tumours
             previously sequenced outside the c-TRAK TN trial must provide one archival tumour
             tissue sample and the report that confirms the mutations detected.

          6. Patients with moderate or high risk early stage triple negative breast cancer
             according to the following risk of relapse criteria:

             Neoadjuvant chemotherapy (no adjuvant chemotherapy planned):

             High risk criteria - Residual microscopic or macroscopic invasive cancer in the
             axillary nodes after chemotherapy Moderate risk criteria - Residual invasive cancer in
             the breast, and axillary lymph node negative after chemotherapy

             Adjuvant chemotherapy:

             High risk criteria - Tumour size >50mm and node positive OR ≥4 nodes positive
             regardless of primary tumour size.

             Moderate risk criteria - Tumour size >20mm AND/OR involved axillary macroscopic lymph
             node.

             Both neoadjuvant and adjuvant chemotherapy:

             Patients who have received both neoadjuvant chemotherapy and further adjuvant
             chemotherapy must fulfil only the adjuvant chemotherapy risk criteria to be eligible.
             They can fulfil the criteria on either clinical staging prior to neoadjuvant
             chemotherapy or pathological staging at surgery.

          7. Patients must be registered according to the following criteria for timing of
             registration:

             Neoadjuvant chemotherapy (no adjuvant chemotherapy planned):

             Patients must be registered within 6 weeks of surgery. Patients may be registered
             before or during radiotherapy and should be registered as early as possible.

             Adjuvant chemotherapy (no neoadjuvant chemotherapy received):

             Patients must be registered before, or on the day of, the 3rd cycle of adjuvant
             chemotherapy and should be registered as early as possible.

             Both neoadjuvant and adjuvant chemotherapy Patients must be registered within 6 weeks
             of surgery. Patients may be registered before or during radiotherapy. Patients must
             register before starting capecitabine.

          8. Consent to provide research blood samples.

          9. Patients with bilateral tumours can be included if both are triple negative and if two
             archival tissues samples can be provided per tumour.

         10. Patients must have had surgery achieving clear margins (as per local guidelines).

         11. Female and male patients of reproductive potential must be willing to use an adequate
             method of contraception, for the first year of the trial and if randomised to
             pembrolizumab, for the duration of treatment through to 120 days after the last dose
             of pembrolizumab. Note: Abstinence is acceptable if this is the usual lifestyle and
             preferred contraception for the patient.

         12. Patients must be willing to have frequent blood tests (every 3 months for 2 years in
             ctDNA screening and 3 weekly if subsequently allocated pembrolizumab) and receive a 12
             month course of pembrolizumab if randomised to pembrolizumab treatment on ctDNA
             detection.

         13. No evidence of distant metastatic disease on staging scans conducted at any time since
             initial diagnosis

        NB: Additional eligibility criteria apply to confirm eligibility to commence pembrolizumab
        treatment following randomisation.

        Exclusion Criteria:

          1. Any concurrent or planned treatment for the current diagnosis of breast cancer other
             than surgery, locoregional adjuvant radiotherapy, standard neoadjuvant or adjuvant
             chemotherapy, or a bisphosphonate/denosumab.

          2. Prior treatment with a programmed death ligand 1(PDL1), programmed cell death protein
             1 (PD1), or other immunomodulatory therapy.

          3. Prior diagnosis of cancer including prior diagnosis of breast cancer in the previous 5
             years, other than for basal cell carcinoma of the skin or cervical carcinoma in situ

          4. Patients previously entered into a therapeutic trial during or after neoadjuvant
             chemotherapy where experimental therapy is continued post-surgery.

          5. Treatment with an unlicensed or investigational product within 4 weeks of trial entry.

          6. Active autoimmune disease requiring systemic therapy in the last two years (i.e. with
             use of disease modifying agents, corticosteroids or immunosuppressive drugs).
             Replacement therapy (e.g. thyroxine, insulin or physiologic corticosteroid replacement
             therapy for adrenal or pituitary insufficiency) is not considered a form of such
             systemic treatment.

          7. Diagnosis of immunodeficiency or receiving systemic steroid therapy or any other form
             of immunosuppressive therapy within 7 days prior to the first dose of pembrolizumab.

          8. Known history of active Tuberculosis Bacillus (TB).

          9. Known history of Human Immunodeficiency Virus (HIV).

         10. Known active Hepatitis B or Hepatitis C.

         11. Known history of, or any evidence of active, non-infectious pneumonitis.

         12. Active infection requiring systemic therapy.

         13. Previous solid organ or allogenic stem cell transplantation

         14. Females who are pregnant or breastfeeding.

         15. Presence of any systemic illness incompatible with participation in the clinical trial
             or inability to provide written informed consent.

        NB. Additional exclusion criteria apply to confirm eligibility to commence pembrolizumab
        treatment following randomisation.