Inclusion Criteria:

          1. Willing and able to provide written informed consent.

          2. Ability to comply with the protocol.

          3. Age ≥ 18 years.

          4. Histopathologically confirmed transitional cell carcinoma. Patients with mixed
             histologies are required to have a dominant (i.e. 50% at least) transitional cell
             pattern.

          5. Fit and planned for cystectomy (according to local guidelines).

          6. Clinical stage T2-T4a N0 M0 disease by CT (or MRI) + PET/CT (within 4 weeks of
             randomization by RECIST v1.1).

          7. Residual disease after TURB (surgical opinion, cystoscopy or radiological presence).

          8. Representative formalin-fixed paraffin-embedded (FFPE) tumor specimens (blocks
             preferred) or at least 15 unstained slides, with an associated pathology report, for
             testing at the study sponsor site and determined to be evaluable for tumor PD-L1
             expression prior to study enrolment; patients with fewer than 15 unstained slides
             available at baseline (but no fewer than 10) may be eligible following discussion with
             Merck representatives.

          9. Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1.

         10. Adequate hematologic and end-organ function tests.

        Exclusion Criteria:

          -  Patients taking regular oral steroids, above the allowed limit of 10mg/day
             methylprednisolone or analogues, for any reason. Patients must not have had steroids
             for 28 days prior to study entry.

          -  Previously intravenous chemotherapy bladder cancer. Patients who have previously had
             radiotherapy or concurrent chemo-radiation would be eligible.

          -  Malignancies other than UBC within 5 years prior to Cycle 1, Day 1, with the exception
             of those with a negligible risk of metastasis or death and treated with expected
             curative outcome (such as adequately treated carcinoma in situ of the cervix, basal or
             squamous cell skin cancer, or ductal carcinoma in situ treated surgically with
             curative intent) or localized prostate cancer treated with curative intent and absence
             of prostate-specific antigen (PSA) relapse or incidental prostate cancer (Gleason
             score ≤ 3 + 4 and PSA < 10 ng/mL undergoing active surveillance and treatment naive).

          -  Evidence of measurable nodal or metastatic disease.

          -  Evidence of significant uncontrolled concomitant disease that could affect compliance
             with the protocol or interpretation of results, including significant liver disease
             (such as cirrhosis, uncontrolled major seizure disorder, or superior vena cava
             syndrome).

          -  Pregnant female patients. All female patients of childbearing potential with a
             positive pregnancy test within 2 weeks prior to the first dose of study treatment will
             be excluded from the study.

          -  Significant cardiovascular disease, such as New York Heart Association cardiac disease
             (Class II or greater), myocardial infarction within 3 months prior to enrolment,
             unstable arrhythmias, or unstable angina.

          -  Severe infections within 4 weeks prior to enrolment in the study including but not
             limited to hospitalization for complications of infection, bacteraemia, or severe
             pneumonia.

          -  Major surgical procedure within 4 weeks prior to enrolment or anticipation of need for
             a major surgical procedure during the course of the study other than for diagnosis.

          -  History of severe allergic, anaphylactic, or other hypersensitivity reactions to
             chimeric or humanized antibodies or fusion proteins

          -  Known hypersensitivity or allergy to biopharmaceuticals produced in Chinese hamster
             ovary cells or any component of the pembrolizumab formulation

          -  History of autoimmune disease including but not limited to myasthenia gravis,
             myositis, autoimmune hepatitis, systemic lupus erythematosus, rheumatoid arthritis,
             inflammatory bowel disease, vascular thrombosis associated with antiphospholipid
             syndrome, Wegener's granulomatosis, Sjögren's syndrome, Guillain-Barré syndrome,
             multiple sclerosis, vasculitis, or glomerulonephritis.

          -  Patients with a history of autoimmune-related hypothyroidism, unless on a stable dose
             of thyroid-replacement hormone.

          -  Patients with uncontrolled Type 1 diabetes mellitus

          -  Uncontrolled hypercalcemia

          -  Patients with prior allogeneic stem cell or solid organ transplantation.

          -  History of idiopathic pulmonary fibrosis

          -  Positive test for HIV.

          -  Patients with active hepatitis infection

          -  Patients with active tuberculosis.

          -  Prior treatment with anti-programmed death-1 (PD-1), or anti-PD-L1 therapeutic
             antibody or pathway-targeting agents.

          -  Administration of a live, attenuated vaccine within 4 weeks prior to enrolment or
             anticipation that such a live, attenuated vaccine will be required during the study

          -  Treatment with any other investigational agent or participation in another clinical
             trial with therapeutic intent within 28 days prior to enrolment

          -  History of severe immune-related adverse effects from anti-CTLA-4 (CTCAE Grade 3 and
             4).

          -  Treatment with systemic immunostimulatory agents within 4 weeks or five half-lives of
             the drug, whichever is shorter, prior to enrolment.