Inclusion Criteria:

          -  Has previously untreated non-small cell lung cancer (NSCLC) diagnosed by histology or
             cytology and confirmed as Stage I or IIA NSCLC (American Joint Committee on Cancer,
             AJCC) by chest computed tomography (CT) and positron emission tomography (PET) scan

          -  Cannot undergo thoracic surgery due to existing medical illness(es) as determined by
             the site's multi-disciplinary tumor board. Medically operable participants who decide
             to treat with stereotactic body radiotherapy (SBRT) as definitive therapy rather than
             surgery are also eligible, if patient's unwillingness to undergo surgical resection is
             clearly documented

          -  Has a Eastern Cooperative Oncology Group (ECOG) Performance Status of 0, 1, or 2

          -  Is able to receive SBRT and does not have an ultra-centrally located tumor

          -  Has adequate organ function within 7 days prior to the start of study treatment

          -  A female is eligible to participate if she is not pregnant, not breastfeeding, and at
             least one of the following conditions applies: a) not a women of childbearing
             potential (WOCBP) OR b) A WOCBP who agrees to use study-acceptable contraception
             during treatment and for at least 120 days after last dose of study treatment

          -  Has a radiation therapy plan approved by the central radiation therapy quality
             assurance vendor

        Exclusion Criteria:

          -  Has received prior therapy with an anti-programmed cell death-1 (anti-PD-1),
             anti-programmed cell death-ligand 1 (anti-PD-L1), or anti PD-L2 agent or with an agent
             directed to another stimulatory or co-inhibitory T-cell receptor (e.g. cytotoxic
             T-lymphocyte-associated antigen 4 [CTLA-4], tumor necrosis factor receptor superfamily
             member 4 [OX-40], tumor necrosis factor receptor superfamily member 9 [CD137])

          -  Has received prior radiotherapy to the thorax, including radiotherapy to the
             esophagus, mediastinum, or breast

          -  Has received a live vaccine within 30 days prior to the first dose of study treatment

          -  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 treatment. 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

          -  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 treatment

          -  Has a known additional malignancy that is progressing or has required active treatment
             within the past 3 years. A prior NSCLC that occurred and was treated curatively at
             least 2 years prior to the date of the current diagnosis would be considered a
             separate primary lung cancer, and therefore an additional malignancy. Note:
             Participants with basal cell carcinoma of the skin, squamous cell carcinoma of the
             skin, or carcinoma in situ (e.g. breast c carcinoma, cervical cancer in situ) that
             have undergone potentially curative therapy are not excluded.

          -  Has a known hypersensitivity (≥Grade 3) to pembrolizumab and/or any of its excipients

          -  Has a history of (non-infectious) pneumonitis that required steroids or has current
             pneumonitis

          -  Has a known history of Hepatitis B or known active Hepatitis C virus infection

          -  Has an active autoimmune disease that has required systemic treatment in past 2 years
             (i.e., with use of disease modifying agents, corticosteroids or immunosuppressive
             drugs). However, replacement therapy (e.g. thyroxine, insulin, or physiologic
             corticosteroid replacement therapy for adrenal or pituitary insufficiency), while
             systemic, will be permitted for study eligibility.

          -  Has an active infection requiring systemic therapy

          -  Has a known history of human immunodeficiency virus (HIV) infection

          -  Has a known history of active tuberculosis (TB; Bacillus tuberculosis)

          -  Is pregnant or breastfeeding or expecting to conceive or father children within the
             projected duration of the study, starting with the screening visit through 120 days
             after the last dose of study treatment

          -  Has had an allogenic tissue/solid organ transplant