This Phase II, double-blind, randomized, placebo-controlled multicenter study will investigate the efficacy and safety of trastuzumab emtansine in combination with atezolizumab or atezolizumab-placebo in participants with HER2-positive locally advanced or metastatic BC who have received prior trastuzumab and taxane based therapy, either alone or in combination, and/or who have progressed within 6 months after completing adjuvant therapy.