Inclusion Criteria:

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

          -  Life expectancy of at least 12 weeks

          -  Diagnosis of R/R MM for which no established therapy for MM is appropriate and
             available, or intolerance to those established therapies

          -  Agreement to provide bone marrow biopsy and aspirate samples

          -  Resolution of adverse events from prior anti-cancer therapy to Grade <=1

          -  Measurable disease

        Exclusion Criteria:

          -  Prior treatment with cevostamab or another agent with the same target

          -  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
             as anti-cancer therapy within 4 weeks before first study treatment, except for the use
             of non-myeloma therapy

          -  Prior treatment with systemic immunotherapeutic agents, including, but not limited to,
             cytokine therapy and anti-CTLA4, anti-PD-1, and antiPD-L1 therapeutic antibodies
             within 12 weeks or 5 half-lives of the drug, whichever is shorter, before first study
             treatment

          -  Prior treatment with chimeric antigen receptor T (CAR T)-cell therapy within 12 weeks
             before first cevostamab infusion

          -  Prior allogeneic stem cell transplant(ation) (SCT)

          -  Known treatment-related, immune-mediated adverse events associated with prior
             checkpoint inhibitors

          -  Current or past history of central nervous system (CNS) disease, such as stroke,
             epilepsy, CNS vasculitis, neurodegenerative disease, or CNS involvement by MM

          -  Significant cardiovascular disease

          -  Symptomatic active pulmonary disease or requiring supplemental oxygen

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

          -  Known or suspected chronic active Epstein-Barr virus (EBV) infection

          -  Recent major surgery within 4 weeks prior to first study treatment

          -  Positive serologic or PCR test results for acute or chronic hepatitis B virus (HBV)
             infection

          -  Acute or chronic hepatitis C virus (HCV) infection

          -  Known history of HIV seropositivity

          -  Administration of a live, attenuated vaccine within 4 weeks before first study
             treatment or anticipation that such a live attenuated vaccine will be required during
             the study

          -  Treatment with systemic immunosuppressive medications, with the exception of
             corticosteroid treatment <=10 mg/day prednisone or equivalent, within 2 weeks prior to
             first study treatment

          -  History of illicit drug or alcohol abuse within 12 months prior to screening, in the
             investigator's judgment