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Inclusion Criteria:

* Archival tumor samples must be obtained from primary and/or metastatic sites
* Able to submit tumor tissue that is evaluable for programmed death- ligand 1 (PD-L1) expression
* HER-2 positive BC as defined by an immunohistochemistry score of 3 or gene amplified by in-situ hybridization as defined by a ratio of greater than or equal to (\>=) 2.0 for the number of HER2 gene copies to the number of chromosome 17 copies
* Histologically or cytologically confirmed invasive BC: incurable, unresectable, locally advanced BC previously treated with multimodality therapy or metastatic BC
* Prior treatment for BC in the: adjuvant; unresectable locally advanced; or metastatic settings; which must include both, a taxane and trastuzumab (alone or in combination with another agent)
* Progression must have occurred during or after most recent treatment for locally advanced/metastatic BC or within 6 months after completing adjuvant therapy
* Participants must have measurable disease that is evaluable as per RECIST v1.1
* Eastern Cooperative Oncology Group Performance Status of 0 or 1
* Negative serum pregnancy test within 7 days of enrollment for pre-menopausal women and for women less than 12 months after the onset of menopause
* Use of highly effective method of contraception as defined by the protocol

Exclusion Criteria:

* Prior treatment with trastuzumab emtansine, cluster of differentiation 137 agonists, anti-programmed death-1, or anti-PD-L1 therapeutic antibody or pathway-targeting agents
* Receipt of any anti-cancer drug/biologic or investigational treatment within 21 days prior to Cycle 1 Day 1 except hormone therapy, which can be given up to 7 days prior to Cycle 1 Day 1; recovery of treatment related toxicity consistent with other eligibility criteria
* Radiation therapy within 2 weeks prior to Cycle 1, Day 1
* History of exposure to the cumulative doses of anthracyclines
* History of other malignancy within the previous 5 years, except for appropriately treated carcinoma in situ of the cervix, non-melanoma skin carcinoma, Stage I uterine cancer, or participants who have undergone potentially curative therapy with no evidence of disease and are deemed by the treating physician to be at low risk for recurrence
* Cardiopulmonary dysfunction, symptomatic pleural effusion, pericardial effusion, or ascites
* Participants with severe infection within 4 weeks prior to randomization, including but not limited to hospitalization for complications of infection, bacteremia, or severe pneumonia
* Current severe, uncontrolled systemic disease
* Major surgical procedure or significant traumatic injury within 28 days prior to randomization or anticipation of the need for major surgery during the course of study treatment
* Clinically significant history of liver disease, including cirrhosis, current alcohol abuse, autoimmune hepatic disorders, sclerosis cholangitis or active infection with human immunodeficiency virus, hepatitis B virus, or hepatitis C virus
* Need for current chronic corticosteroid therapy (\>=10 mg of prednisone per day or an equivalent dose of other anti-inflammatory corticosteroids)
* 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 greater than (\>) 2 weeks prior to randomization
* Participants with known central nervous system disease
* Leptomeningeal disease
* History of autoimmune disease
* Prior allogeneic stem cell or solid organ transplantation
* Active tuberculosis
* Receipt of a live, attenuated vaccine within 4 weeks prior to randomization or anticipation that such a live, attenuated vaccine will be required during the study
* Treatment with systemic immunostimulatory agents within 4 weeks or five half-lives of the drug (whichever is shorter) prior to randomization
* Treatment with systemic corticosteroids or other systemic immunosuppressive medications within 2 weeks prior to randomization, or anticipated requirement for systemic immunosuppressive medications during the trial
* Participants who are breastfeeding, or intending to become pregnant during the study

• Orange, California, United States
• Aurora, Colorado, United States
• Washington D.C., District of Columbia, United States
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• Baltimore, Maryland, United States
• Farmington, New Mexico, United States
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• Seattle, Washington, United States
• Kogarah, New South Wales, Australia
• Waratah, New South Wales, Australia
• Woolloongabba, Queensland, Australia
• East Melbourne, Victoria, Australia
• Frankston, Victoria, Australia
• St Albans, Victoria, Australia
• Subiaco, Western Australia, Australia
• Oshawa, Ontario, Canada
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• Toronto, Ontario, Canada
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• Montreal, Quebec, Canada
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• Berlin, Germany
• Berlin, Germany
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