Study of Atezolizumab Plus Carboplatin and Etoposide With or Without Tiragolumab in Participants With Untreated Extensive-Stage Small Cell Lung Cancer

Inclusion Criteria:

* Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1
* Histologically or cytologically confirmed Extensive-Stage Small Cell Lung Cancer (ES-SCLC) per the modified Veterans Administration Lung Study Group (VALG) staging system
* No prior systemic treatment for ES-SCLC
* For participants who have received prior chemoradiotherapy for limited-stage SCLC must have had treatment with curative intent and a treatment-free interval of at least 6 months between the last dose/cycle of chemotherapy, thoracic radiotherapy, or chemoradiotherapy and the diagnosis of ES-SCLC
* Measurable diseases as defined by RECIST v1.1
* Submission of a pre-treatment tumor tissue sample
* Adequate hematologic and end-organ function
* Participants not receiving therapeutic anticoagulation with International Normalized Ratio (INR) and Activated Clotting Time (aPTT) \</= 1.5 x ULN
* Participants receiving therapeutic anticoagulation: stable anticoagulant regimen
* Negative Human Immunodeficiency Virus (HIV) test at screening
* Negative hepatitis B surface antigen (HBsAg) test at screening
* Positive hepatitis B surface antibody (HBsAb) test at screening, or negative HBsAb at screening accompanied by either of the following: negative total hepatitis B core antibody (HBcAb) and/or positive total HBcAb test followed by a negative hepatitis B virus (HBV) DNA test
* Negative hepatitis C virus (HCV) antibody test at screening, or positive HCV antibody test followed by a negative HCV RNA test
* Negative Epstein-Barr virus (EBV) viral capsid antigen (VCA) IgM test or negative EBV polymerase chain reaction (PCR) test at screening
* For women of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraception, and agreement to refrain from donating eggs
* For men: agreement to remain abstinent (refrain from heterosexual intercourse) or use contraceptive methods, and agreement to refrain from donating sperm.

Exclusion Criteria:

* Symptomatic or actively progressing central nervous system (CNS) metastases
* Spinal cord compression
* Leptomeningeal disease
* Uncontrolled pleural effusion, pericardial effusion, or ascites
* Uncontrolled or symptomatic hypercalcemia
* Known clinically significant liver disease, including active viral, alcoholic, or other hepatitis, cirrhosis, and inherited liver disease, or current alcohol abuse
* Malignancies other than SCLC within 5 years prior to randomization
* Active or history of autoimmune disease or immune deficiencies
* History of idiopathic pulmonary fibrosis, organizing pneumonia, drug-induced pneumonitis, or idiopathic pneumonitis, or evidence of active pneumonitis on screening chest Computer Tomography (CT) scan
* Known active tuberculosis, Current treatment with anti-viral therapy for HBV or HCV
* Severe chronic or active infection
* Treatment with therapeutic oral or IV antibiotics
* Significant cardiovascular disease
* Major surgical procedure other than for diagnosis
* Prior allogeneic 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
* Administration of a live, attenuated vaccine
* Prior treatment with CD137 agonists, T-cell co-stimulating, or immune checkpoint blockade therapies
* Treatment with systemic immunostimulatory agents
* Treatment with systemic immunosuppressive medications
* History of severe allergic anaphylactic reactions to chimeric or humanized antibodies or fusion proteins
* Known hypersensitivity to Chinese Hamster Ovary (CHO) cell products or to any component of the tiragolumab or atezolizumab formulations
* History of allergic reactions to carboplatin or etoposide
* Pregnancy or breastfeeding, or intention of becoming pregnant during study treatment or within 5 months after the final dose of atezolizumab or within 90 days after the final dose of tiragolumab or for 6 months after the final dose of carboplatin or etoposide.