Inclusion Criteria:

          -  Histologically or cytologically confirmed extensive-stage small cell lung cancer
             (ES-SCLC)

          -  No prior systemic treatment for ES-SCLC

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

          -  Measurable disease, as defined by Response Evaluation Criteria in Solid Tumors version
             1.1 (RECIST v1.1)

          -  Adequate hematologic and end-organ function

          -  Treatment-free for at least 6 months since last chemo/radiotherapy, among those
             treated (with curative intent) with prior chemo/radiotherapy for limited-stage SCLC

        Exclusion Criteria:

          -  Symptomatic or actively progressing central nervous system (CNS) metastases

          -  Malignancies other than small cell lung cancer (SCLC) within 5 years prior to
             randomization, with the exception of those with a negligible risk of metastasis or
             death treated with expected curative outcome

          -  Active or history of autoimmune disease or immune deficiency

          -  History of idiopathic pulmonary fibrosis, organizing pneumonia, drug-induced
             pneumonitis, or idiopathic pneumonitis, or evidence of active pneumonitis on screening
             chest computed tomography (CT) scan

          -  Positive test result for human immunodeficiency virus (HIV)

          -  Active hepatitis B or hepatitis C

          -  Severe infection at the time of randomization

          -  Treatment with any other investigational agent within 28 days prior to initiation of
             study treatment

          -  Prior treatment with CD137 agonists or immune checkpoint blockade therapies,
             anti-cytotoxic T lymphocyte-associated protein 4 (anti-CTLA-4), anti-TIGIT, anti-PD-1,
             and anti-PD-L1 therapeutic antibodies

          -  Treatment with systemic immunostimulatory agents within 4 weeks or 5 drug elimination
             half-lives prior to randomization