Inclusion Criteria:

   - Metastatic or locally advanced, histologically documented TNBC characterized by
   absence of human epidermal growth factor 2 (HER2), estrogen receptor (ER), and
   progesterone receptor (PR) expression

   - No prior chemotherapy or targeted systemic therapy for inoperable locally advanced or
   metastatic TNBC

   - Eligible for taxane monotherapy (i.e., absence of rapid clinical progression,
   life-threatening visceral metastases, or the need for rapid symptom and/or disease
   control)

   - A representative formalin-fixed, paraffin-embedded tumor specimen in paraffin blocks,
   or at least 20 unstained slides with an associated pathology report documenting ER,
   PR, and HER2 negativity. Participants with fewer than 20 unstained slides available at
   baseline, and not fewer than 12 unstained slides will be eligible upon discussion with
   Medical Monitor

   - Eastern Cooperative Oncology Group performance status of 0 or 1

   - Measurable disease as defined by RECIST v1.1

   - Adequate hematologic and end-organ function

Exclusion Criteria:

   - Known central nervous system (CNS) disease, except for treated asymptomatic CNS
   metastases

   - Leptomeningeal disease

   - Pregnancy or lactation

   - History of autoimmune disease

   - Prior allogeneic stem cell or solid organ transplantation

   - Positive test for human immunodeficiency virus

   - Active hepatitis B or hepatitis C

   - Receipt of a live, attenuated vaccine within 4 weeks prior to randomization, during
   treatment, or within 5 months following the last dose of atezolizumab/placebo