Inclusion Criteria:

          -  Participants should weigh at least 35 kg.

          -  Eastern Cooperative Oncology Group performance status of 0 or 1 and a life expectancy
             of >= 3 months.

          -  Participant have >= 1 lesion accessible for intratumoral (IT) injection.

          -  Histologically or cytologically confirmed R/M HNSCC (of the following 4 subsites: oral
             cavity, oropharynx, larynx, and hypopharynx) who previously progressed either during
             or after <= 2 prior treatment regimens administered in the recurrent or metastatic
             setting.

          -  Must have received 1 immunotherapy regimen which included a PD-(L)1 inhibitor in the
             recurrent or metastatic setting.

        Exclusion Criteria:

          -  No uncontrolled metastases to the central nervous system (CNS).

          -  Participants with brain metastases are eligible provided that evidence of clinical and
             radiographic stable disease for at least 4 weeks after definitive therapy is given and
             participants have not used prohibited levels of steroids for at least 4 weeks prior to
             first dose of the study.

          -  Must not have received any prior anti-cancer immunotherapy other than a programmed
             cell death 1 (PD-1) or programmed cell death ligand 1 (PD-L1) inhibitor.