Inclusion Criteria:

          -  Previously diagnosed with Multiple Myeloma (MM) based on standard criteria.

          -  Dose Escalation Phase: Participants with r/r MM who have previously received therapy
             with an Immunomodulatory drug (IMiD) and Proteasome Inhibitor (PI) and are intolerant
             to or have no other option for standard-of-care treatment according to the
             Investigator.

          -  Dose Expansion Phase: Participants with r/r MM who have received at least three prior
             treatments and are refractory to an IMiD, a PI, and a CD38-targeted therapy.

          -  Life expectancy of at least 12 weeks.

          -  Agreement to provide protocol-specific biopsy material.

          -  AEs from prior anti-cancer therapy resolved to Grade =<1.

          -  Measurable disease.

          -  For female participants of childbearing potential: agreement to remain abstinent
             (refrain from heterosexual intercourse), use contraceptive measures and refrain from
             donating eggs.

          -  For male participants: agreement to remain abstinent (refrain from heterosexual
             intercourse), use contraceptive measures and refrain from donating sperm.

        Exclusion Criteria:

          -  Inability to comply with protocol-mandated hospitalization and activities
             restrictions.

          -  Pregnant or breastfeeding, or intending to become pregnant during the study or within
             3 months after the last dose of study drug.

          -  Prior use of any monoclonal antibody, radioimmunoconjugate, or antibody-drug conjugate
             within 2 weeks before first RO7425781 administration.

          -  Prior treatment with systemic immunotherapeutic agents within 2 weeks before first
             RO7425781 administration.

          -  Treatment-related, immune-mediated AEs associated with prior immunotherapeutic agents.

          -  Treatment with radiotherapy, any chemotherapeutic agent, or treatment with any other
             anti-cancer agent (investigational or otherwise) within 2 weeks except for alkylating
             agents (e.g., melphalan) within 28 days, prior to first RO7425781 administration.
             Limited field palliative radiotherapy for bone pain or for soft tissue lesions is
             allowed.

          -  Autologous or allogeneic stem cell transplantation (SCT) within 100 days prior to
             first RO7425781 administration and/or signs of chronic graft versus host disease or
             ongoing immunosuppressive medication.

          -  Prior solid organ transplantation.

          -  Any medical condition or abnormality in clinical laboratory tests that, in the
             Investigator's or Medical Monitor's judgment, precludes the participant's safe
             participation in and completion of the study, or which could affect compliance with
             the protocol or interpretation of results.