This study will assess the efficacy and safety of capivasertib plus docetaxel versus placebo plus docetaxel in participants with metastatic castration resistant prostate cancer (mCRPC), all participants will receive the docetaxel with steroid therapy and receive androgen deprivation therapy. The intention of the study is to demonstrate that the combination of capivasertib plus docetaxel is superior to placebo plus docetaxel with respect to the overall survival and/or the radiographic progression free survival of study participants.