A Study of Trastuzumab Emtansine in Combination With Atezolizumab or Placebo as a Treatment for Participants With Human Epidermal Growth Factor 2 (HER2)-Positive and Programmed Death-ligand 1 (PD-L1)-Positive Locally Advanced (LABC) or Metastatic Breast Cancer (MBC)

Inclusion Criteria:

• HER2+ and PD-L1+ locally advanced (LABC) or metastatic breast cancer (MBC)

• Progression must have occurred during most recent treatment for LABC/MBC or during,or within 6 months after completing, neoadjuvant and/or adjuvant therapy

• Prior treatment with trastuzumab (+/- pertuzumab) and taxane in the neoadjuvantand/or adjuvant, locally advanced, or metastatic setting

• No more than two prior lines of therapy in the metastatic setting

• Measurable disease per RESIST version 1.1

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

• Life expectancy >= 6 months

• Adequate hematologic and end-organ function

• For women of childbearing potential: agreement to remain abstinent or usecontraception, and agreement to refrain from donating eggs

• For men: agreement to remain abstinent or use contraceptive measures, and agreementto refrain from donating sperm

Exclusion Criteria:

• Prior treatment with trastuzumab emtansine in metastatic setting

• History of exposure to cumulative doses of anthracyclines

• Symptomatic or actively progressing central nervous system (CNS) metastases;asymptomatic CNS lesions ≤ 2cm without clinical requirement for local interventionor asymptomatic patients with treated CNS lesions are eligible

• Current Grade >= 3 peripheral neuropathy

• Cardiopulmonary dysfunction

• History of malignancy within 5 years prior to initiation of study treatment, withthe exception of the cancer under investigation and malignancies with a negligiblerisk of metastasis or death

• History of leptomeningeal disease

• Active or history of autoimmune disease or immune deficiency

• Active hepatitis B, hepatitis C and/or tuberculosis

• Prior allogeneic stem cell or solid organ transplantation

• Receipt of a live, attenuated vaccine within 4 weeks prior to initiation of studytreatment, during treatment, or within 5 months following the last dose of studytreatment

• Pregnancy or lactation