This is a Phase III, randomized, multicenter, multinational, two-arm, open-label clinical trial to investigate a first-line treatment of participants with human epidermal growth factor receptor-2 (HER2)-positive metastatic breast cancer. The study will enroll patients with HER2-positive, unresectable, locally advanced breast cancer (BC) if they have recurrent disease or progressive disease (PD) despite primary multi-modality therapy, and/or metastatic BC if they have not received prior chemotherapy for their metastatic disease. Eligible participants at up to approximately 40 sites in the Asia-Pacific region will be randomized in a 2:1 ratio to receive trastuzumab emtansine (Arm A) or trastuzumab plus docetaxel (Arm B). All study drugs will be administered at in-clinic visits occurring every three weeks during the treatment phase. Trastuzumab plus docetaxel was chosen as the comparator in the control group (Arm B), as it represents a common first-line treatment option used in this patient population in China and other Asia-Pacific countries.