Small cell lung cancer (SCLC) is characterized by aggressive and rapid growth and a tendency to develop early spread to distant sites including mediastinal lymph nodes, liver, bones, adrenal glands, and brain. The purpose of this study is to assess safety, dose, change in disease activity of ABBV-706 given with atezolizumab, compared to standard of care (SOC) treatment (etoposide, carboplatin, atezolizumab, and optional lurbinectedin). ABBV-706 is an investigational drug being developed for the treatment of SCLC. There are multiple treatment arms in this study. Participants will either receive ABBV-706 given with atezolizumab, at 1 of 2 doses, or SOC. Approximately 180 adult participants will be enrolled in the study across sites worldwide. In the safety lead-in, participants with SCLC will receive intravenous (IV) ABBV-706 in 1 of 2 doses with IV atezolizumab, or IV SOC. In the expansion portion of the study, participants with SCLC will receive IV ABBV-706 in 1 of 2 doses with atezolizumab, or IV SOC, until the optimal dose of ABBV-706 is determined. The estimated duration of the study is up to 69.5 months. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic and may require frequent medical assessments, blood tests, questionnaires, and scans.