The current study will compare pembrolizumab (MK-3475) plus maintenance olaparib, versus (vs) pembrolizumab plus maintenance pemetrexed for the treatment of non-squamous NSCLC. The study's 2 primary hypotheses are: 1. Pembrolizumab plus maintenance olaparib is superior to pembrolizumab plus maintenance pemetrexed with respect to progression-free survival (PFS) per Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST 1.1) by blinded independent clinical review (BICR) and 2. Pembrolizumab plus maintenance olaparib is superior to pembrolizumab plus maintenance pemetrexed with respect to overall survival (OS).

This study has 2 phases: an Induction Phase of up to \~4 Cycles (up to \~12 weeks \[cycle =3 weeks\]) and a Maintenance Phase of up to \~31 cycles of pembrolizumab (cycle = 3 weeks\]); total pembrolizumab treatment duration will be up to \~35 cycles (up to \~2 years). In the Induction Phase, participants receive pembrolizumab plus pemetrexed plus platinum (carboplatin or cisplatin, at the investigator's discretion). In the Maintenance Phase, participants with a partial or complete disease response or with stable disease after completing four cycles of induction therapy and who meet eligibility criteria will be randomly assigned to receive pembrolizumab plus maintenance olaparib OR pembrolizumab plus maintenance pemetrexed. In the Maintenance Phase, participants receive pembrolizumab for up to 31 cycles (cycle = 3 weeks) plus maintenance olaparib OR maintenance pemetrexed until progressive disease (PD), intolerable toxicities, or physician decision. Qualified participants in each study arm of the maintenance phase (Pembrolizumab plus Olaparib and Pembrolizumab plus Pemetrexed) who complete up to \~35 cycles of pembrolizumab (up to \~2 years \[cycle =3 weeks\]) may be eligible to receive a second course of pembrolizumab for up to \~17 cycles (up to \~1 additional year). Per protocol, response or progression during the second pembrolizumab course will not be counted towards patient reported outcomes (PROs) or efficacy outcome measures and adverse events during the second pembrolizumab course will not be counted towards safety outcome measures.