Condition: High-risk Non-muscle Invasive Bladder Cancer

This study is designed to assess the antitumor efficacy and safety of pembrolizumab in combination with BCG, compared to BCG monotherapy, in participants with HR NMIBC that is either persistent or recurrent following adequate BCG induction (Cohort A), or that is naïve to BCG treatment (Cohort B). The primary hypothesis for Cohort A is that the combination of pembrolizumab plus BCG has a superior complete response rate (CRR) as assessed by central pathology review compared to BCG in participants with carcinoma in situ (CIS). The primary hypothesis for Cohort B is that the combination of pembrolizumab plus BCG (either reduced maintenance or full maintenance) has a superior Event Free Survival (EFS) compared to BCG.