A Clinical Study to Evaluate the Effects of RO7875913 in Healthy Participants and of RO7875913 With Cevostamab in Participants With Relapsed/Refractory Multiple Myeloma

Part A: The purpose of Part A of this study is to evaluate the safety, pharmacokinetics, and pharmacodynamics of RO7875913 in healthy participants. Part B: The purpose of Part B of this study is to evaluate the safety, pharmacokinetics, pharmacodynamics, and preliminary clinical activity of RO7875913 administered in combination with the T cell-engaging bispecific antibody (TCB) cevostamab in participants with relapsed or refractory (R/R) multiple myeloma (MM).

Inclusion Criteria: General: * Agreement to adhere to the contraception requirements Part A: * Body weight \> 40 kilogram (kg) with a body mass index of 18-30 kg per meter square (kg/m\^2) Part B: * Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 or 1 * Life expectancy of at least 12 weeks * Agreement to provide bone marrow biopsy and aspirate samples Exclusion Criteria: General: * Treatment with any vaccine within 4 weeks prior to initiation of study drug, or vaccination scheduled to occur during the study * History or current cardiovascular or pulmonary disease that may limit the ability to respond to systemic infusion/injection reactions * Positive test result for hepatitis B surface antigen, hepatitis C virus (HCV), or human immunodeficiency virus (HIV) antibody screen * History of any malignancy * Major surgical procedure within 28 days prior to initiation of study treatment, or anticipation of need for a major surgical procedure during the study * History or clinical manifestations of significant metabolic, hepatic, renal, pulmonary, cardiovascular, hematologic, gastrointestinal, urologic, neurologic, or psychiatric disorders * Known allergy or hypersensitivity to any component of the RO7875913 formulation Part A: * Treatment with investigational biologic therapy (or blinded comparator) within 90 days or 5 drug elimination half-lives, whichever is longer, prior to initiation of study drug * Treatment with investigational non-biologic therapy (or blinded comparator) within 28 days or 5 drug elimination half-lives, whichever is longer, prior to initiation of study drug * Clinically apparent or familial history of autoimmune disease Part B: * Treatment with any systemic chemotherapeutic agent, or treatment with any other anti-cancer agent (investigational or otherwise) within 4 weeks or 5 half-lives of the drug, whichever is shorter, prior to first dose of study treatment * Treatment with any immunosuppressive medication within 2 weeks prior to first dose of study treatment * Absolute plasma cell count exceeding 500/mL or 5% of the peripheral blood white cells