Key Inclusion Criteria:

          -  Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1

          -  Histologically or cytologically confirmed diagnosis of squamous cell carcinoma of the
             esophagus

          -  Unresectable locally advanced disease

          -  Definitive concurrent chemoradiation treatment according to regional oncology
             guidelines for esophageal cancer

          -  Representative archival formalin-fixed, paraffin-embedded (FFPE) tumor specimens,
             collected prior to initiation of definitive chemoradiotherapy

          -  Adequate hematologic and end-organ function

          -  Female participants must be willing to avoid pregnancy and refrain from donating eggs
             during the treatment period and for 90 days after the final dose

          -  Male participants with partners of childbearing potential must commit to the use of
             two methods of contraception and must not donate sperm for the study duration and 90
             days after the final dose

        Key Exclusion Criteria:

          -  Prior treatment with CD137 agonists or immune checkpoint blockade therapies, including
             anti-CTLA-4, anti-PD-1, anti-PD-L1 and anti-TIGIT therapeutic antibodies

          -  Any unresolved toxicity of National Cancer Institute's (NCI) Common Terminology
             Criteria for Adverse Events (CTCAE) Grade ≥ 2 from the prior chemoradiation therapy
             with the exception of irreversible and manageable hearing loss

          -  Prior allogeneic stem cell or solid organ transplantation

          -  Active or history of autoimmune disease or immune deficiency

          -  History of idiopathic pulmonary fibrosis, organizing pneumonia (e.g., bronchiolitis
             obliterans), drug-induced pneumonitis, or idiopathic pneumonitis, or evidence of
             active pneumonitis

          -  Malignancies other than esophageal cancer within 2 years prior to screening, with the
             exception of malignancies with a negligible risk of metastasis or death

          -  Treatment with any other investigational agent, including epidermal growth factor
             receptor (EGFR) inhibitors, with therapeutic intent for esophageal cancer prior to
             randomization