Inclusion Criteria:

        For the purposes of this study, neoadjuvant and/or adjuvant chemotherapy regimens do not
        count as a prior line of therapy.

        For second line plus monotherapy (Parts 1 and 2):

          -  Has received at least one systemic treatment for metastatic breast cancer

          -  Has documented disease progression on or after the most recent therapy

          -  Prior treatment must include an anthracycline and a taxane in the neoadjuvant,
             adjuvant, or metastatic setting

        For first line monotherapy (Part 1):

          -  Has received no prior systemic treatment for metastatic breast cancer

          -  Has PD-L1-positive mTNBC.

        For second line plus monotherapy (Part 2):

        - Has PD-L1 strong positive mTNBC

        For all parts:

          -  Has mTNBC confirmed by a central laboratory

          -  For biomarker analysis, adequate newly obtained core or excisional biopsy of a
             not-previously-irradiated metastatic tumor lesion (mandatory)

          -  Has measurable metastatic disease

          -  Has Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1

          -  Female participants of childbearing potential should be willing to use 2 methods of
             birth control or be surgically sterile, or abstain from heterosexual activity for the
             course of the study through 120 days after the last dose of study treatment

          -  Male participants should agree to use an adequate method of contraception starting
             with the first dose of study treatment through 120 days after the last dose of study
             treatment

          -  Has adequate organ function

        Exclusion Criteria:

          -  Is currently participating and receiving study treatment, or has participated in a
             study of an investigational agent and received study therapy or used an
             investigational device within 4 weeks prior to study Day 1

          -  Has received prior anti-cancer monoclonal antibody (mAb) therapy for direct
             anti-neoplastic treatment within 4 weeks prior to study Day 1

          -  Has received prior chemotherapy, targeted small molecule therapy, or radiation therapy
             within at least 2 weeks prior to study Day 1

          -  Has not recovered (i.e., ≤ Grade 1 or at baseline) from adverse events due to agents
             administered within at least 2 weeks prior to study Day 1

          -  Has an active autoimmune disease requiring systemic treatment in past 2 years

          -  Has a diagnosis of immunodeficiency or receiving systemic steroid therapy or any other
             form of immunosuppressive therapy within 7 days prior to the first dose of study
             treatment

          -  Has known additional malignancy that progressed or required active treatment within
             the last 5 years. Exceptions include basal cell carcinoma of the skin, squamous cell
             carcinoma of the skin that has undergone potentially curative therapy, or in situ
             cervical cancer

          -  Has radiographically-detectable central nervous system (CNS) metastases and/or
             carcinomatous meningitis

          -  Has a history of (non-infectious) pneumonitis that required steroids or current
             pneumonitis or a history of interstitial lung disease

          -  Has an active infection requiring systemic therapy

          -  Has known psychiatric or substance abuse disorders that would interfere with
             cooperation with the requirements of the study

          -  Is pregnant, breastfeeding, or expecting to conceive or father children within the
             projected duration of the study, starting with the screening visit through 120 days
             after the last dose of study treatment

          -  Has received prior therapy with an anti-programmed cell death protein-1 (anti-PD-1),
             anti-PD-L1, anti-PD-L2 agent or with an agent directed to another co-inhibitory T-cell
             receptor (e.g. cytotoxic T-lymphocyte-associated protein-4 [CTLA-4], OX-40, CD137) or
             has participated in Merck MK-3475 (pembrolizumab) study

          -  Has a known history of human immunodeficiency virus (HIV)

          -  Has known active Hepatitis B or C

          -  Has received a live vaccine within 30 days of planned start of study treatment