...Cohorts 1 and 1A (combination of niraparib and PD-1 inhibitor): participants must have tumors with high PD-L1 expression (TPS >= 50%) per local assessment; with no known estimated glomerular filtration rate (EGFR)-sensitizing mutation and/or ROS -1 or anaplastic lymphoma kinase (ALK) translocation, and no prior systemic chemotherapy or PD-1/PD-L1 inhibitor treatment for metastatic NSCLC...Cohorts 2 and 2A (combination of niraparib and PD-1 inhibitor): participants must have tumors with PD-L1 expression (TPS between 1% and 49%) per local assessment, with no known EGFR-sensitizing mutation and/or ROS-1 or ALK translocation, and no prior systemic chemotherapy or PD-1/PD-L1 inhibitor treatment for metastatic NSCLC (all histologies)...Stage 1: Objective response rate (ORR) in participants with NSCLC whose tumors have high PD-L1 expression (TPS: >=50%)...Stage 2: Objective response rate (ORR) in participants with NSCLC whose tumors have high PD-L1 expression (TPS: >=50%)...