This study is for people with high-risk non-muscle invasive bladder cancer (HR NMIBC). HR NMIBC is a type of bladder cancer that is found in the bladder's lining but hasn't spread to the muscle or outside the bladder. High-risk means it might get worse or come back after treatment. The study aims to see if a medicine called pembrolizumab, when used with Bacillus Calmette-Guerin (BCG), helps more people live longer without cancer compared to using BCG alone. It will also check if this combination is safe and tolerable.

• Study Duration: Treatment and follow-up can last several months.
• Study Visits: Participants will need regular visits for treatment and monitoring.
• Potential Risks: Participants need to meet strict health criteria and should be aware of possible side effects.

Eligible participants should have had a specific type of HR NMIBC diagnosis and surgery to remove any visible cancer. They must agree to use contraception if they are of childbearing potential. People with certain health conditions, like other cancers or infections, may not be able to join.