A Phase 3 Study of Pembrolizumab (MK-3475) in Combination with Concurrent Chemoradiation Therapy Followed by Pembrolizumab with or without Olaparib vs Concurrent Chemoradiation Therapy Followed by Durvalumab in Participants with Unresectable, Locally Advanced, Stage III Non-Small Cell Lung Cancer (NSCLC)

1. Has previously untreated, unresectable, histologically or cytologically confirmed stage IIIA, IIIB, or IIIC NSCLC by American Joint Committee on Cancer Version 8 that is considered candidate for definitive concurrent chemoradiation. Note: Sites are encouraged to obtain tissue confirmation of mediastinal nodal involvement if this is safely accessible by endobronchial ultrasound (EBUS)/endoscopic ultrasound (EUS) or mediastinoscopy. However, mediastinal nodal involvement can be declared by imaging when the nodes have distinct margins and the size of the shortest axis of at least 1 node is ≥2.0 cm (positron emission tomography [PET]/magnetic resonance imaging [MRI]/computed tomography [CT] scan). Cases in which the distinct nodes all have short axis of < 2.0 cm must have pathologic confirmation of disease presence for participants to be declared eligible. 2. Participants with cervical nodal involvement are not permitted. Participants with supraclavicular nodal involvement may be entered into the study. The upper border of supraclavicular nodes must not extend above the upper border of the lateral end of the clavicle, extended medially. 3. Has no evidence of metastatic disease by protocol-approved imaging studies. The process for image collection and transmission to the central imaging vendor can be found in the Site Imaging Manual. NOTE: The presence of pleural/pericardial fluid is presumed indicative of metastatic disease unless proven otherwise. A pleural effusion fluid that is present on both a CT chest scan and routine chest x-ray requires a pleuracentesis to confirm the effusion is cytologically negative. Participants with effusions that are exudates are excluded even if the effusions are cytologically negative. Participants with effusions that are not visible on routine chest x-ray or are too small to be tapped safely may be entered on this study, provided the remaining inclusion/exclusion criteria are met. 4. Has at least 1 lesion that meets the criteria for being measurable as defined by RECIST 1.1, and is appropriate for selection as a target lesion, as determined by local site investigator/radiology review. Lesions that appear measurable, but have undergone palliative irradiation, cannot be target lesions. 5. Has not received prior systemic treatment for their Stage III NSCLC. Participants who received systemic therapy for earlier stages are eligible if the therapies were completed at least 12 months prior to the development of stage III NSCLC. Note: Participants must have recovered from all AEs due to previous therapies to ≤Grade 1 or baseline. Participants with ≤Grade 2 neuropathy may be eligible. 6. Has provided tumor sample (tissue biopsy [core, incisional, or excisional]). Formalin-fixed, paraffin embedded (FFPE) blocks are preferred to slides. Newly obtained tumor sample is highly preferred over archival tissue. Note: If submitting unstained cut slides, newly cut slides must be submitted to the testing laboratory within 14 days from the date slides are cut (details pertaining to tumor tissue submission can be found in the Procedures Manual). 7. Has a performance status of 0 or 1 on the Eastern Cooperative Oncology Group (ECOG) Performance Status assessed within 7 days prior to the first administration of study intervention. 8. Has a life expectancy of at least 6 months. 9. Has adequate pulmonary function test (PFT) defined as a forced expiratory volume in 1 second (FEV1) > 50% of predicted normal volume and the carbon monoxide lung diffusing capacity (DLCO) > 40% of predicted normal value. Participants for whom DLCO measurements are not available will be deemed to have adequate oxygen transfer if their resting capillary/arterial blood gas on room air reveals an oxygen pressure (PO2) > 60 mmHg. 10. Has adequate organ function as defined in Table 1; all screening laboratory tests should be performed within 10 days prior to initiation of study treatment. Table 1 Adequate Organ Function Laboratory Values System Laboratory Value Hematological ANC ≥1500/µL Platelets ≥100 000/µL Hemoglobin ≥9.0 g/dL or ≥5.6 mmol/La Renal Estimated creatinine clearance using the Cockcroft-Gault equation testb or a 24-hour urine test ≥51 mL/min Hepatic Total bilirubin ≤1.5 ×ULN OR direct bilirubin within normal limits for participants with total bilirubin levels > 1.5 × ULN AST (SGOT) and ALT (SGPT) ≤2.5 × ULN Endocrine TSH Within normal limits. Note: If TSH is not within normal limits at baseline, the participant may still be eligible if T3 and free T4 are within the normal limits Coagulation INR OR PT aPTT OR PTT ≤1.5 × ULN unless participant is receiving anticoagulant therapy as long as PT or aPTT is within therapeutic range of intended use of anticoagulant Note: This table includes eligibility-defining laboratory value requirements for treatment; laboratory value requirements should be adapted according to local regulations and guidelines for the administration of specific chemotherapies. Demographics 11. Is male or female of at least18 years of age inclusive, at the time of signing the informed consent. Male Participants Contraceptive use by men should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies. 12. A male participant must agree to use contraception as detailed in Appendix 5 of this protocol during the treatment period and for at least 180 days following the last dose of study treatment. Note: Male participants should refrain from donating sperm during the treatment period and for at least 180 days following the last dose of study treatment. Female Participants Contraceptive use by women should be consistent with local regulations regarding the methods of contraception for those participating in clinical studies. 13. A female participant is eligible to participate if she is not pregnant (Appendix 5), not breastfeeding, and at least 1 of the following conditions applies: a. Not a woman of childbearing potential (WOCBP) as defined in Appendix 5. OR A WOCBP who agrees to follow the contraceptive guidance in Appendix 5 during the treatment period and for at least 180 days following the last dose of pembrolizumab, olaparib, placebo or durvalumab or at least 180 days following the last dose of cytotoxic chemotherapy. Informed Consent 14. Has (or legally acceptable representative if applicable) provided written informed consent/assent for the study. The participant may also provide consent/assent for Future Biomedical Research. However, the participant may participate in the main trial without participating in Future Biomedical Research.

Medical Conditions 1. Has small cell lung cancer or a mixed tumor with presence of small cell elements. Note: Participants with squamous NSCLC are not eligible to receive pemetrexed based chemotherapy. 2. Has history, current diagnosis, or features suggestive of myelodysplastic syndrome (MDS)/acute myeloid leukemia (AML). 3. Has had documented weight loss > 10% in the preceding 3 months. Prior/Concomitant Therapy 4. Has a radiation treatment plan that is likely to encompass a volume of whole lung receiving > 20 Gy in total (V20) of more than 35% of lung volume. 5. Has received prior radiotherapy to the thorax, including radiotherapy to the esophagus, mediastinum, or for breast cancer. 6. Completed palliative radiotherapy within 7 days of the first dose of trial treatment. Participants must have recovered from all radiation-related toxicities, not require corticosteroids for at least 7 days prior to the first dose of study intervention, and not have had radiation pneumonitis. The radiated lesion must not be included as a target lesion for RECIST 1.1 measurements. 7. Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti PD-L2 agent or with an agent directed to another stimulatory or co-inhibitory T-cell receptor (eg, CTLA-4, OX-40, CD137). 8. Has received prior therapy with olaparib or with any other PARP inhibitor 9. Had major surgery < 4 weeks prior to the first dose of study drug (except for placement of vascular access). Note: If participants received major surgery, or underwent placement of vascular access, they must have recovered adequately from the toxicity and/or complications from the intervention prior to starting study intervention. 10. Is expected to require any other form of antineoplastic therapy, including radiation therapy, while on study. 11. Has received a live vaccine within 30 days prior to the first dose of study drug. Examples of live vaccines include, but are not limited to, the following: measles, mumps, rubella, varicella/zoster (chicken pox), yellow fever, Bacillus Calmette-Guérin (BCG), and typhoid vaccine. Seasonal influenza vaccines for injection are generally killed virus vaccines and are allowed; however, intranasal influenza vaccines (eg, FluMist®) are live attenuated vaccines and are not allowed. 12. Has received colony-stimulating factors (e.g., granulocyte colony-stimulating factor [G-CSF], granulocyte-macrophage colony-stimulating factor [GM-CSF] or recombinant erythropoietin) within 28 days prior to the first dose of study intervention. 13. Is currently receiving either strong (phenobarbital, enzalutamide, phenytoin, rifampicin, rifabutin, rifapentine, carbamazepine, nevirapine and St John’s Wort) or moderate (e.g. bosentan, efavirenz, modafinil) inducers of CYP3A4 that cannot be discontinued for the duration of the study. The required washout period prior to starting olaparib is 5 weeks for pentobarbital and 3 weeks for other agents. Note: a current list of strong/moderate inducers of CYP3A4 can be found at the following website: https://www.fda.gov/Drugs/DevelopmentApprovalProcess/DevelopmentResources/DrugInteractionsLabeling/ucm093664.htm 14. Is currently receiving either strong (eg, itraconazole, telithromycin, clarithromycin, protease inhibitors boosted with ritonavir or cobicistat, indinavir, saquinavir, nelfinavir, boceprevir, telaprevir) or moderate (eg. ciprofloxacin, erythromycin, diltiazem, fluconazole, verapamil) inhibitors of cytochrome P450 (CYP)3A4 that cannot be discontinued for the duration of the study. The required washout period prior to starting olaparib is 2 weeks. Note: a current list of strong/moderate inhibitors of CYP3A4 can be found at the following website: https://www.fda.gov/Drugs/DevelopmentApprovalProcess/DevelopmentResources/DrugInteractionsLabeling/ucm093664.htm Pemetrexed-specific 15. Is unable to interrupt aspirin or other nonsteroidal anti-inflammatory drugs (NSAIDs), other than an aspirin dose ≤1.3 grams per day, for at least 2 days (5 days for long-acting agents [for example, piroxicam]) before, during, and for at least 2 days after administration of pemetrexed. 16. Is unable/unwilling to take folic acid, vitamin B12, and dexamethasone. Prior/Concurrent Clinical Study Experience 17. Is currently participating in or has participated in a study of an investigational agent or has used an investigational device within 4 weeks prior to the first dose of study intervention. Note: Participants who have entered the follow-up phase of an investigational study may participate as long as it has been 4 weeks after the last dose of the previous investigational agent. Diagnostic Assessments 18. Has resting electrocardiogram (ECG) indicating uncontrolled, potentially reversible cardiac conditions, as judged by the Investigator (eg, unstable ischemia, uncontrolled symptomatic arrhythmia, congestive heart failure, electrolyte disturbances, etc.), or participant has congenital long QT syndrome. 19. Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy (in dosing exceeding 10 mg daily of prednisone equivalent) or any other form of immunosuppressive therapy within 7 days prior the first dose of study drug. Note: ocular, intranasal, or topical steroids or inhaled corticosteroids for management of asthma or chronic obstructive pulmonary disease are permitted. 20. Has a known additional malignancy that is progressing or has required active treatment within the past 5 years. Note: Participants with basal cell carcinoma of the skin, squamous cell carcinoma of the skin, superficial bladder cancer, or carcinoma in situ (eg, breast carcinoma, cervical cancer in situ) that have undergone potentially curative therapy are not excluded. 21. Has severe hypersensitivity (≥ Grade 3) to study treatment and/or any of its excipients. 22. Has an active autoimmune disease that has required systemic treatment in past 2 years (ie, with use of disease modifying agents, corticosteroids or immunosuppressive drugs). Replacement therapy (eg, thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency) is not considered a form of systemic treatment and is allowed. 23. Has a known history of, or active, neurologic paraneoplastic syndrome. 24. Has a history of (non-infectious) pneumonitis that required steroids or has current pneumonitis. 25. Has an active infection requiring systemic therapy. 26. Has a known history of human immunodeficiency virus (HIV) infection. No HIV testing is required unless mandated by local health authority. 27. Has a known history of Hepatitis B (defined as HBsAg reactive) or known active Hepatitis C virus (defined as HCV RNA [qualitative] is detected) infection. Note: No testing for Hepatitis B and Hepatitis C is required unless mandated by local health authority 28. Has a known history of active tuberculosis (TB; Mycobacterium tuberculosis). 29. Has a known history of interstitial lung disease. Lymphangitic spread of the NSCLC is not exclusionary. 30. Has a history or current evidence of any condition, therapy, or laboratory abnormality that might confound the results of the study, interfere with the participant's participation for the full duration of the study, or is not in the best interest of the participant to participate, in the opinion of the treating investigator. 31. In the opinion of the treating Investigator, is considered a poor medical risk due to a serious, uncontrolled medical disorder or non-malignant systemic disease. Examples include, but are not limited to, uncontrolled ventricular arrhythmia, recent (within 3 months) myocardial infarction, uncontrolled major seizure disorder, unstable spinal cord compression, or superior vena cava syndrome. 32. Has a known psychiatric or substance abuse disorder that would interfere with the participant’s ability to cooperate with the requirements of the study. Other Exclusions 33. Is unable to swallow orally administered medication or has a gastrointestinal disorder affecting absorption (e.g. gastrectomy, partial bowel, obstruction, malabsorption). 34. Is pregnant or breastfeeding or expecting to conceive or father children within the projected duration of the study, starting with the screening visit through 180 days after the last dose of study intervention.