Inclusion Criteria:

        Dose Escalation, Maintenance Arm A:

          -  Participants with ES-SCLC who have completed 4-6 cycles of carboplatin and etoposide
             induction chemotherapy, with or without atezolizumab, as their first-line therapy for
             extensive-stage disease and have responded (CR or PR) or have Stable Disease (SD) are
             eligible for the maintenance arm of the study.

          -  All side effects attributed to prior anti-cancer therapy must have resolved to Grade 1
             or baseline.

          -  A maximum of 8 weeks (56 days) is allowed between last chemotherapy dose (Cycle 4, Day
             3) given in induction and the start of maintenance therapy.

        Dose Escalation, Induction Arm B:

          -  Participants with no prior systemic treatment for ES-SCLC are eligible for this study.

          -  ANC >= 1,500 cells/µL without granulocyte colony-stimulating factor support.

        Dose Expansion, Maintenance-Only:

          -  Participants with ES-SCLC who have completed 4 cycles of carboplatin and etoposide
             induction chemotherapy and at least 3 cycles of atezolizumab as their first-line
             therapy for extensive-stage disease and have responded (CR or PR) or have SD are
             eligible for the maintenance arm of the study.

        Dose Escalation (Arms A and B) and Dose Expansion:

          -  Ability to comply with the study protocol, in the investigator's judgement.

          -  ECOG performance status of 0 or 1.

          -  Participants must be able to swallow pills.

          -  Histologically or cytologically confirmed diagnosis of ES-SCLC per the Veterans
             Administration Lung Study Group (VALG) staging system.

          -  Participants who received prior chemoradiotherapy for limited-stage SCLC must have
             been treated with curative intent and experienced a treatment-free interval of at
             least 6 months since last chemotherapy, radiotherapy or chemoradiotherapy cycle prior
             to diagnosis of ES-SCLC.

          -  Participants with a history of treated CNS metastases that are currently asymptomatic.

          -  Measurable disease, as defined by RECIST v1.1. Baseline measurements and evaluation of
             all sites of disease must be obtained =<4 weeks prior to enrollment.

          -  Eligible to receive a carboplatin-based chemotherapy regimen.

          -  Adequate hematologic and end-organ function.

          -  Participants must submit a pre-treatment tumor tissue sample.

          -  Participants must submit a blood sample for exploratory biomarker research before
             treatment, on-study, and following progression of disease.

          -  For women of childbearing potential: agreement to remain abstinent (refrain from
             heterosexual intercourse), use non-hormonal contraceptive methods and refrain from
             donating eggs.

          -  Women who are not postmenopausal (>=12 months of non-therapy-induced amenorrhea) or
             surgically sterile must have a negative serum pregnancy test result within 14 days
             prior to initiation of study drug.

          -  For men: agreement to remain abstinent (refrain from heterosexual intercourse) or use
             a condom and agreement to refrain from donating sperm.

        Exclusion Criteria:

          -  Use of non-protocol-specified anti-cancer therapies or other combination partners with
             carboplatin/etoposide during induction.

          -  Symptomatic or actively progressing CNS metastases.

          -  Pregnant or breastfeeding, or intending to become pregnant during the study.

          -  Spinal cord compression not definitively treated with surgery and/or radiation or
             previously diagnosed and treated spinal cord compression without evidence that disease
             has been clinically stable for >= 1 week prior to enrollment.

          -  Leptomeningeal disease.

          -  Uncontrolled pleural effusion, pericardial effusion or ascites requiring recurrent
             drainage procedures (once a month or more frequently).

          -  Uncontrolled or symptomatic hypercalcemia.

          -  History of malignancy other than SCLC within 5 years prior to enrollment.

          -  History of autoimmune disease.

          -  History of idiopathic pulmonary fibrosis, organizing pneumonia (e.g., bronchiolitis
             obliterans), drug-induced pneumonitis, idiopathic pneumonitis, or evidence of active
             pneumonitis on screening chest CT scan.

          -  Positive HIV infection.

          -  Active Hepatitis B and C infection (HBV/HCV).

          -  Active Tuberculosis infection.

          -  Known infection with human T-cell leukemia virus 1.

          -  Known active bacterial, viral, fungal, mycobacterial, parasitic, or other infection
             (excluding fungal infections of nail beds) at study enrollment, or any major episode
             of infection requiring treatment with IV antibiotics or hospitalization.

          -  Significant cardiovascular disease.

          -  Major surgical procedure within 28 days prior to enrollment or anticipation of need
             for major surgical procedure during the course of the study.

          -  Prior allogenic bone marrow transplantation or solid organ transplant.

          -  Any other diseases, metabolic dysfunction, physical examination finding, or clinical
             laboratory finding giving reasonable suspicion of a disease or condition that
             contraindicates the use of an investigational drug or that may affect the
             interpretation of the results or render the participant at high risk for treatment
             complications.

          -  Illnesses or conditions that interfere with their capacity to understand, follow,
             and/or comply with study procedures.

          -  Treatment with investigational therapy with therapeutic intent within 28 days prior to
             enrollment.

          -  Administration of a live, attenuated vaccine within 4 weeks before enrollment or
             anticipation that such a live attenuated vaccine will be required during the study.

          -  Prior treatment with CD137 agonists or immune checkpoint blockade therapies,
             anti-PD-1, and anti-PD-L1 therapeutic antibodies.

          -  Treatment with systemic immunosuppressive medications within 1 week prior to
             enrollment.

          -  History of severe allergic, anaphylactic, or other hypersensitivity reactions to
             chimeric or humanized antibodies or fusion proteins.

          -  Known hypersensitivity to biopharmaceuticals produced in Chinese hamster ovary cells
             or any component of the atezolizumab formulation.

          -  History of allergic reactions to carboplatin or etoposide or to any of its excipients
             (etoposide).

          -  Known hypersensitivity to venetoclax or to any of its excipients.

          -  Administration of Steroid therapy for anti-neoplastic intent, strong or moderate CYP3A
             inhibitors or strong or moderate CYP3A inducers within 7 days prior to the first dose
             of study drug.

          -  Consumption of grapefruit, grapefruit products, Seville oranges (including
             marmalade-containing Seville oranges), or starfruit (carambola) within 3 days prior to
             the first dose of study drug.

          -  Malabsorption syndrome or other condition that would interfere with enteral
             absorption.

          -  Illicit drug or alcohol abuse within 12 months prior to screening, in the
             investigator's judgement.

          -  Inability or unwillingness to swallow a large number of tablets.

          -  History of inflammatory bowel disease (e.g., Crohn's disease or ulcerative colitis) or
             active bowel inflammation (e.g., diverticulitis).