Inclusion Criteria: - Histologically or cytologically documented extensive-stage small cell lung cancer (ES-SCLC) and extensive-stage disease (American Joint Committee on Cancer, 8th edition, Stage IV [T any, N any, M1a, M1b, or M1c], or T3-4 due to multiple lung nodules that are too extensive or tumor or nodal volume that is too large to be encompassed in a tolerable radiation plan) - Participants taking part in the separate PET tracer sub-study must provide a fresh tumor biopsy from any disease site (primary or metastatic) - Archived tumor specimens, in the form of blocks or sectioned slides, are mandatory for all participants except those participating in the separate PET tracer sub-study for whom the archived tumor specimen is optional - Eastern Cooperative Oncology Group performance status (ECOG PS) 0 or 1 - At least 1 measurable lesion by computed tomography (CT) or magnetic resonance imaging (MRI) per Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST v1.1) criteria - Adequate hematologic and end organ function - Must agree to follow specific methods of contraception, if applicable Exclusion Criteria: - Women who are pregnant or breastfeeding. Japan only: participation in the study is not allowed even if breastfeeding is suspended - Prior chemotherapy, radiation therapy, or biologic therapy for SCLC. Previously treated limited stage SCLC (LS-SCLC) participants are also excluded - Symptomatic brain or other central nervous system (CNS) metastases - Paraneoplastic autoimmune syndrome requiring systemic treatment - History of idiopathic pulmonary fibrosis, drug-induced pneumonitis, idiopathic pneumonitis, organizing pneumonia, or evidence of active pneumonitis on screening chest CT scan - Grade ≥ 2 peripheral sensory neuropathy at study entry - Significant uncontrolled cardiovascular disease - Active, known or suspected autoimmune disease or inflammatory disorder Other protocol-defined inclusion/exclusion criteria apply