Inclusion Criteria for the Surveillance Phase:

          -  Histologically confirmed MIUC (also termed TCC) of the bladder

          -  TNM classification (based on AJCC Cancer Staging Manual, 7th Edition; Edge et al.
             2010) at pathological examination of surgical resection specimen as follows: For
             patients treated with prior NAC: tumor stage of ypT2-4a or ypN+ and M0. For patients
             who have not received prior NAC: tumor stage of pT3-4a or pN+ and M0

          -  Surgical resection of MIUC of the bladder

          -  Patients who have not received prior platinum-based NAC, have refused, or are
             ineligible ("unfit") for cisplatin-based adjuvant chemotherapy

          -  ctDNA assay developed based on tumor tissue specimen and matched normal DNA from
             blood.

          -  Tumor PD-L1 expression per IHC and confirmed diagnosis of MIUC as documented through
             central testing of a representative tumor tissue specimen

          -  Absence of residual disease and absence of metastasis, as confirmed by a negative
             baseline computed tomography (CT) or magnetic resonance imaging (MRI) scan of the
             pelvis, abdomen, and chest no more than 4 weeks prior to enrollment.

          -  Full recovery from cystectomy and enrollment within 14 weeks following cystectomy.
             Minimum of 6 weeks must have elapsed from surgery.

        Additional Inclusion Criteria for the Treatment Phase:

          -  Plasma sample evaluated to be ctDNA positive

          -  Absence of residual disease and absence of metastasis, as confirmed by a negative
             baseline CT or MRI scan of the pelvis, abdomen, and chest no more than 4 weeks prior
             to randomization.

          -  ECOG Performance Status of <= 2

          -  Life expectancy >=12 weeks

          -  Adequate hematologic and end-organ function

          -  For women of childbearing potential: agreement to remain abstinent (refrain from
             heterosexual intercourse) or use contraception and agreement to refrain from donating
             eggs

        General Medical Exclusion Criteria:

          -  Pregnancy or breastfeeding

          -  History of severe allergic, anaphylactic, or other hypersensitivity reactions to
             chimeric or humanized antibodies or fusion proteins

          -  Known hypersensitivity to biopharmaceuticals produced in Chinese hamster ovary cells
             or any component of the atezolizumab formulation

          -  History of autoimmune disease. Patients with a history of autoimmune-related
             hypothyroidism on a stable dose of thyroid replacement hormone may be eligible for
             this study. Patients with controlled Type I diabetes mellitus on a stable dose of
             insulin regimen may be eligible for this study.

          -  History of idiopathic pulmonary fibrosis, organizing pneumonia, drug-induced
             pneumonitis, idiopathic pneumonitis, or evidence of active pneumonitis on screening
             chest CT scan. History of radiation pneumonitis in the radiation field (fibrosis) is
             permitted.

          -  Positive test for HIV

          -  Patients with active hepatitis B virus or hepatitis C

          -  Active tuberculosis

          -  Significant cardiovascular disease, such as New York Heart Association cardiac disease
             (Class II or greater), myocardial infarction within the previous 3 months, unstable
             arrhythmias, or unstable angina

        Cancer-Specific Exclusion Criteria:

          -  Any approved anti-cancer therapy, including chemotherapy, or hormonal therapy within 3
             weeks prior to study enrollment

          -  Adjuvant chemotherapy or radiation therapy for UC following cystectomy

          -  Treatment with any other investigational agent or participation in another clinical
             trial with therapeutic intent within 28 days or 5 half-lives of the drug, whichever is
             longer, prior to enrollment

          -  Malignancies other than UC within 5 years prior to study enrollment

        Additional Exclusion Criteria for the Treatment Phase:

          -  Any approved anti-cancer therapy, including chemotherapy, or hormonal therapy within 3
             weeks prior to randomization to the treatment phase Hormone-replacement therapy or
             oral contraceptives are allowed.

          -  Adjuvant chemotherapy or radiation therapy for UC following cystectomy

          -  Treatment with any other investigational agent or participation in another clinical
             trial with therapeutic intent within 28 days or 5 half-lives of the drug, whichever is
             longer, prior to randomization to the treatment phase