Safety and Pharmacology Study of Atezolizumab Alone and in Combination With Bacille Calmette-Guérin (BCG) in High-Risk Non-Muscle-Invasive Bladder Cancer (NMIBC) Participants

Inclusion Criteria: * Histologically confirmed non-muscle-invasive transitional cell carcinoma (TCC) of the bladder with carcinoma in-situ (CIS) * High-risk NMIBC defined by the following: BCG-unresponsive NMIBC: Persistence of high-grade CIS at 6 months following an adequate course of BCG; or Stage/grade progression at 3 months after induction BCG; or Recurrence of high-grade CIS after achieving a disease-free state (i.e., CR) following induction of an adequate course of BCG that occurs less than (\<) 6 months after the last exposure to BCG BCG-relapsing NMIBC: Recurrence of high-grade CIS after achieving a disease-free state following induction of an adequate course of BCG that occurs greater than or equal to (\>/=) 6 months after the last exposure to BCG Very high-risk (VHR) BCG-naïve NMIBC: VHR NMIBC, defined as having at least 1 of the following: Multiple and/or large (greater than \[\>\] 3 centimeters \[cm\]) T1, (HG/G3) tumors; T1, (HG/G3) tumor with concurrent CIS; T1, G3 with CIS in prostatic urethra; Micropapillary variant of non-muscle invasive urothelial carcinoma * For BCG-unresponsive and BCG-relapsing NMIBC, participants must have received an adequate course of BCG * Resection of all pTa/pT1 papillary disease * No prior radiation to bladder or pelvic region * Eastern Cooperative Oncology Group (ECOG) performance status of less than or equal to (\</=) 2; * Life expectancy \>/=12 weeks * Adequate hematologic and end-organ function * Creatinine clearance \>/=30 milliliters per minute (mL/min) (calculated using the Cockcroft-Gault formula) * For women of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive methods that result in a failure rate of \<1% per year during the treatment period and for at least 5 months after the last dose of study drug. Women must refrain from donating eggs during this same period. * For men receiving BCG: Agreement to remain abstinent (refrain from sexual intercourse) or use a condom * Tumor tissue biopsy within 60 days prior to study entry or availability of an archival specimen obtained within 60 days of study screening Exclusion Criteria: * Evidence of locally advanced, metastatic, muscle-invasive, and/or extravesical bladder cancer * Any malignancy within 5 years prior to Cycle 1, Day 1 * History of autoimmune disease, idiopathic pulmonary fibrosis, organizing pneumonia (e.g., bronchiolitis obliterans), drug-induced pneumonitis, idiopathic pneumonitis, or active pneumonitis * Signs or symptoms of infection within 2 weeks prior to the first dose of study treatment * Treatment with therapeutic oral or IV antibiotics within 2 weeks prior to the first dose of study treatment * Treatment with any approved anti-cancer therapy within 3 weeks prior to the first dose of study treatment * Treatment with any other investigational agent or participation in another clinical trial with therapeutic intent within 4 weeks prior to the first dose of study treatment * Pregnant or lactating women, or women intending to become pregnant during the study * Prior allogeneic stem cell or solid organ transplantation * Positive test for human immunodeficiency virus (HIV) * Active hepatitis B or C and/or tuberculosis * Severe infections within 28 days prior to the first dose of study treatment * Significant cardiovascular disease * Major surgical procedure other than for diagnosis within 4 weeks prior to the first dose of study treatment, or anticipation of need for a major surgical procedure during the course of the study * Administration of a live/attenuated vaccine within 4 weeks prior to the first dose of study treatment, within 5 months following the administration of the last dose of study drug, or anticipation that such a live/attenuated vaccine will be required during the study * History of prior significant toxicity or intolerance to BCG requiring discontinuation of treatment * History of prior systemic BCG infection * History of immunosuppression, or conditions associated with congenital or acquired immune deficiency * Concurrent febrile illness, urinary tract infection, or gross hematuria * Prior treatment with cluster of differentiation 137 (CD137) agonists or immune checkpoint blockade therapies * Treatment with systemic immunostimulatory agents within 6 weeks or five half-lives of the drug, whichever is shorter, prior to the first dose of study treatment * Treatment with systemic immunosuppressive medications within 2 weeks prior to the first dose of study treatment, or anticipated requirement for systemic immunosuppressive medications during the trial