A Study to Evaluate the Efficacy and Safety of Trastuzumab Emtansine in Combination With Atezolizumab or Atezolizumab-Placebo in Participants With Human Epidermal Growth Factor-2 (HER2) Positive Locally Advanced or Metastatic Breast Cancer (BC) Who Received Prior Trastuzumab and Taxane Based Therapy

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