Key Inclusion Criteria:

          -  ECOG PS of 0, 1, or 2

          -  Histologically confirmed FL, DLBCL, or MCL

          -  Must have received at least one prior systemic treatment regimen containing an
             anti-CD20-directed therapy for DLBCL or FL

          -  For MCL, participants must have received at least two prior systemic treatment
             regiments, which include 1) anti-CD20-directed therapy, 2) BTK inhibitor, and 3)
             anthracycline or bendamustine

          -  Relapsed to prior regimen(s) after having a documented history of response (complete
             response [CR], CR unconfirmed [CRu], or partial response [PR]) of >/= 6 months in
             duration from completion of regimen(s); or, refractory to any prior regimen, defined
             as no response to the prior therapy, or progression within 6 months of completion of
             the last dose of therapy

          -  Measurable disease, defined as at least one bi-dimensionally measurable nodal lesion,
             defined as > 1.5 cm in its longest dimension, or at least one bi-dimensionally
             measurable extranodal lesion, defined as > 1.0 cm in its longest dimension

          -  Adequate hematologic, renal, and hepatic function

        Key Exclusion Criteria:

          -  Prior treatment with mosunetuzumab or other CD20-directed bispecific antibodies

          -  Prior treatment with polatuzumab vedotin

          -  Current > Grade 1 peripheral neuropathy

          -  Prior use of any monoclonal antibody, radioimmunoconjugate or antibody-drug conjugate
             (ADC) within 4 weeks before first dose of study treatment

          -  Treatment with any 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 radiotherapy within 2 weeks prior to the first dose of study treatment

          -  Autologous stem-cell transplantation (SCT) within 100 days prior to first study
             treatment administration

          -  Prior treatment with chimeric antigen receptor T-cell (CAR-T) therapy within 30 days
             before first study treatment administration

          -  Prior allogeneic SCT

          -  Prior solid organ transplantation

          -  Known or suspected history of hemophagocytic lymphohistiocytosis (HLH)

          -  Patients with history of confirmed progressive multifocal leukoencephalopathy (PML)

          -  Current or past history of central nervous system (CNS) lymphoma or CNS disease

          -  History of autoimmune disease