Inclusion Criteria:

        General Inclusion Criteria:

          -  Part A: Patient must have histologically or cytologically confirmed advanced and/or
             metastatic solid tumor malignancies for which standard curative or palliative measures
             do not exist, are no longer effective, or are not acceptable to the patient

          -  Eastern Cooperative Oncology Group Performance Status 0-1

          -  Measurable disease as defined by Response Evaluation Criteria in Solid Tumors (RECIST
             v1.1)

          -  Fresh biopsies may be required

          -  Negative HIV, hepatitis B, or hepatitis C test result

          -  Women of childbearing potential and male participants must agree to remain abstinent
             or use contraceptive methods as defined by the protocol

        Additional Specific Inclusion Criteria for Participants with Melanoma:

          -  Histologically confirmed, unresectable stage III or stage IV melanoma

          -  Previously treated with approved anti-programmed death-ligand 1
             (PD-L1)/anti-programmed death-1 (PD-1) agents with or without approved anti-cytotoxic
             T-lymphocyte-associated protein 4 (CTLA-4) therapy and up to one additional treatment
             regimen

        Additional Specific Inclusion Criteria for Participants with Non-small Cell Lung Cancer
        (NSCLC) who Previously Received Treatment for Metastatic Disease:

          -  Histologically confirmed advanced NSCLC

          -  Previously treated with approved PD-L1/PD-1 inhibitors and platinum-based chemotherapy

          -  Not more than 2 prior lines of treatment for metastatic disease are allowed prior to
             enrolling to the study

          -  Participants must have experienced initial clinical benefit (stable disease or better)
             from most recent checkpoint inhibitor (CPI) therapy

          -  Tumor PD-L1 expression as determined by immunohistochemistry assay of archival tumor
             tissue or tissue obtained at screening

        Additional Specific Inclusion Criteria for Participants with Non-small Cell Lung Cancer
        (NSCLC) who Previously Did Not Receive Treatment for Metastatic Disease:

          -  Histologically confirmed advanced NSCLC

          -  Tumor PD-L1 expression as determined by immunohistochemistry assay of archival tumor
             tissue or tissue obtained at screening

        Additional Specific Inclusion Criteria for Participants with Small Cell Lung Cancer (SCLC):

          -  Histologically confirmed SCLC

          -  Participants may have had prior chemotherapy, radiation therapy, or declined approved
             therapies for SCLC

        Additional Specific Inclusion Criteria for Participants with Esophageal Squamous Cell
        Carcinoma (ESCC):

          -  Participants whose major lesion was histologically confirmed as squamous cell
             carcinoma or adenosquamous cell carcinoma of the esophagus

          -  Patients who have previously received not more than 1 prior line of treatment for
             metastatic disease prior to enrolling to the study

        Exclusion Criteria:

        General Exclusion Criteria:

          -  Pregnancy, lactation, or breastfeeding

          -  Known hypersensitivity to any of the components of RO7121661

          -  Active or untreated central nervous system (CNS) metastases

          -  An active second malignancy

          -  Evidence of concomitant diseases, metabolic dysfunction, physical examination
             findings, or clinical laboratory findings giving reasonable suspicion of a disease or
             condition that contraindicates the use of an investigational drug or that may affect
             the interpretation of the results or render the participant at high risk from
             treatment complications

          -  Known active or uncontrolled bacterial, viral, fungal, mycobacterial, parasitic, or
             other infection

          -  Treatment with oral or IV antibiotics within 2 weeks prior to Cycle 1 Day 1

          -  Active or history of autoimmune disease or immune deficiency

          -  Prior treatment with adoptive cell therapies, such as CAR-T therapies

          -  Concurrent therapy with any other investigational drug <28 days or 5 half-lives of the
             drug, whichever is shorter, prior to the first RO7247669 administration

          -  Regular immunosuppressive therapy

          -  Radiotherapy within the last 4 weeks before start of study drug treatment, with the
             exception of limited palliative radiotherapy

          -  Prior treatment with a T-cell immunoglobulin and mucin domain-3 (TIM-3) inhibitor

        Additional Specific Exclusion Criteria for Participants with NSCLC who Previously Received
        Treatment for Metastatic Disease:

        - Patients with the following mutations, rearrangements, translocations are not eligible:
        epidermal growth factor receptor (EGFR); anaplastic lymphoma kinase (ALK); ROS
        proto-oncogene 1 (ROS1), BRAFV600E, and neurotrophic receptor tyrosine kinase (NTRK)

        Additional Specific Exclusion Criteria for Participants with NSCLC who Did Not Previously
        Receive Treatment for Metastatic Disease:

          -  Prior therapy for metastatic disease

          -  Adjuvant anti-PD-1 or anti-PD-L1 therapy

        Additional Specific Exclusion Criteria for Participants with Small-Cell Lung Cancer (SCLC):

        - Prior therapy with any immune CPIs (such as anti-PD-L1/PD-1, CTLA-4)

        Additional Specific Exclusion Criteria for Participants with Esophageal Squamous Cell
        Carcinoma (ESCC):

        - Prior therapy with any immunomodulatory agents