Inclusion Criteria

          -  Eastern Cooperative Oncology Group (ECOG) Performance Status of 0, 1, or 2

          -  R/R FL after treatment with at least one prior chemo immunotherapy regimen that
             included an anti CD20 monoclonal antibody (MAb)

          -  Histologically documented FL of Grade 1, 2, or 3a, and that expresses CD20 at time of
             diagnosis as determined by the local laboratory

          -  Fluorodeoxyglucose avid lymphoma (i.e., positron emission tomography (PET) positive
             lymphoma)

          -  At least one bi dimensionally measurable nodal lesion (>1.5 cm in its largest
             dimension by PET- computed tomography (CT) scan), or at least one bi dimensionally
             measurable extranodal lesion (>1.0 cm in its largest dimension by PET-CT scan)

          -  Availability of a representative tumor specimen and the corresponding pathology report
             for confirmation of the diagnosis of FL

          -  Adequate hematologic function (unless due to underlying lymphoma, per the
             investigator) as defined by the protocol

          -  Normal laboratory values (unless due to underlying lymphoma) as defined by the
             protocol

          -  Agreement to comply with all local requirements of the Len risk minimization plan

          -  For women of childbearing potential: agreement to remain abstinent or use two adequate
             methods of contraception, including at least one method with a failure rate of < 1%
             per year, for at least 28 days prior to Day 1 of Cycle 1, during the treatment period,
             and for at least 12 months after the final dose of glofitamab, 28 days after the last
             dose of Len, 18 months after the last dose of G, 3 months after the final dose of
             tocilizumab, and 3 months after the final dose of Mosun. Women must refrain from
             donating eggs during this same period

          -  For men: agreement to remain abstinent or use contraceptive measures and agreement to
             refrain from donating sperm, with female partners of childbearing potential or
             pregnant female partners, men must remain abstinent or use a condom during the
             treatment period and for at least 2 months after the final dose of glofitamab, 28 days
             after last dose of Len, 18 months after the last dose of G, 3 months after the final
             dose of tocilizumab, and 3 months after the final dose of Mosun

        Exclusion Criteria

          -  Grade 3b FL

          -  History of transformation of indolent disease to diffuse large B-cell lymphoma (DLBCL)

          -  Documented refractoriness to an obinutuzumab monotherapy containing regimen in
             glofitamab-containing treatment combination

          -  Active or history of central nervous system (CNS) lymphoma or leptomeningeal
             infiltration

          -  Documented refractoriness to lenalidomide, defined as no response (partial response
             (PR) or complete response (CR)) within 6 months of therapy

          -  Prior standard or investigational anti-cancer therapy as specified by the protocol

          -  Clinically significant toxicity (other than alopecia) from prior treatment that has
             not resolved to Grade <=2 prior to Day 1 of Cycle 1

          -  Known history of idiopathic pulmonary fibrosis, organizing pneumonia (e.g.
             bronchiolitis obliterans), drug-induced pneumonitis or evidence of active pneumonitis
             on screening chest CT scan

          -  Treatment with systemic immunosuppressive medications, including, but not limited to,
             prednisone, azathioprine, methotrexate, thalidomide, and anti-tumor necrosis factor
             agents within 2 weeks prior to Day 1 of Cycle 1

          -  History of solid organ transplantation

          -  History of severe allergic or anaphylactic reaction to humanized, chimeric or murine
             MAbs

          -  Known sensitivity or allergy to murine products

          -  Known hypersensitivity to biopharmaceuticals produced in Chinese hamster ovary cells
             or any component of the glofitamab, Mosun, G, Len, or thalidomide formulation,
             including mannitol

          -  History of erythema multiforme, Grade >=3 rash, or blistering following prior
             treatment with immunomodulatory derivatives

          -  Known history of idiopathic pulmonary fibrosis, organizing pneumonia, drug-induced
             pneumonitis or evidence of active pneumonitis on screening chest CT scan

          -  Known active bacterial, viral, fungal, or other infection, or any major episode of
             infection requiring treatment with IV antibiotics within 4 weeks of Day 1 of Cycle 1

          -  Known or suspected chronic active Epstein-Barr virus infection or hemophagocytic
             syndrome

          -  Known history of macrophage activating syndrome (MAS) or hemophagocytic
             lymphohistiocytosis (HLH)

          -  Active Hepatitis B and Hepatitis C infection or autoimmune disease requiring treatment

          -  Prior allogenic hematopoietic stem cell transplant

          -  Known history of HIV positive status

          -  History of progressive multifocal leukoencephalopathy

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

          -  Other malignancy that could affect compliance with the protocol or interpretation of
             results

          -  Prior allogenic hematopoietic stem cell transplant (HSCT)

          -  Contraindication to treatment for thromboembolism prophylaxis

          -  Grade >=2 neuropathy

          -  Evidence of any significant, uncontrolled concomitant disease that could affect
             compliance with the protocol or interpretation of results, including, but not limited
             to significant cardiovascular disease or significant pulmonary disease

          -  Major surgical procedure other than for diagnosis within 28 days prior to Day 1 of
             Cycle 1 Day 1 or anticipation of a major surgical procedure during the course of the
             study

          -  Clinically significant history of liver disease, including viral or other hepatitis,
             current alcohol abuse, or cirrhosis

          -  Inadequate hematologic function

          -  Any of the following abnormal laboratory values

          -  Pregnant or lactating or intending to become pregnant during the study

          -  Life expectancy < 3 months

          -  Unable to comply with the study protocol, in the investigator's judgment

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

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