Inclusion Criteria

          -  Confirmed locally advanced and/or metastatic solid tumor, with at least one tumor
             lesion of accessible non-critical location to biopsy, in participants who have
             progressed on a standard therapy, are intolerant to standard therapy, and/or are
             non-amenable to standard therapy

          -  Radiologically measurable and clinically evaluable disease (as per RECIST v1.1)

          -  Life expectancy (in the opinion of the investigator) of at least 12 weeks and lactate
             dehydrogenase (LDH) levels </= 2.5 ULN (upper limit of normal)

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

          -  All acute toxic effects of any prior radiotherapy, chemotherapy, or surgical procedure
             must have resolved to Grade </= 1 or returned to baseline except alopecia (any grade)
             and Grade 2 peripheral neuropathy

          -  Adequate hematological, liver, and renal function

          -  Negative serum pregnancy test within 7 days prior to study treatment in premenopausal
             women and women </= 2 years after start of menopause (menopause is defined as
             amenorrhea for more than 2 years)

          -  Participants must agree to remain abstinent or be willing to use effective methods of
             contraception as defined in the protocol

          -  Participants with non-colorectal cancer should have confirmed CEA expression in tumor
             tissue. For colorectal cancer (CRC), the CEA assessment should be performed but the
             result is not required for participant selection

        Exclusion Criteria

          -  Active or untreated central nervous system (CNS) metastases as determined by computed
             tomography (CT) or magnetic resonance imaging (MRI) evaluation during screening and
             prior radiographic assessments

          -  Spinal cord compression not definitively treated with surgery and/or radiation or
             previously diagnosed and treated spinal cord compression without evidence that disease
             has been clinically stable for at least 2 weeks prior to enrollment

          -  Leptomeningeal disease

          -  Participants with paraspinal, paratracheal, and mediastinal pathological lesions
             larger than 2 cm unless they are previously irradiated

          -  Malignancies within 5 years prior to enrollment, with the exception of those with a
             negligible risk of metastasis or death and treated with expected curative outcome

          -  Significant, uncontrolled concomitant diseases which could affect compliance with the
             protocol or interpretation of results

          -  Uncontrolled hypertension, unstable angina, congestive heart failure (CHF), serious
             cardiac arrhythmia requiring treatment history of myocardial infarction within 6
             months of enrollment

          -  Administration of a live, attenuated vaccine within 28 days before Cycle 1 Day 1 or
             anticipation that such a live attenuated vaccine will be required during the study

          -  Human Inmmunodeficiency Virus (HIV), active Hepatitis B or Hepatitis C (HCV)

          -  Severe infections within 28 days prior to Cycle 1 Day 1, including but not limited to
             hospitalization for complications of infection, bacteremia, or severe pneumonia or
             active tuberculosis

          -  Received oral or intravenous (IV) antibiotics within 14 days prior to Day 1

          -  Any other diseases, metabolic dysfunction, physical examination finding, or clinical
             laboratory finding giving reasonable suspicion of a disease or condition that would
             contraindicate the use of an investigational drug

          -  Major surgery or significant traumatic injury less than 28 days prior to Cycle 1 Day 1
             (excluding biopsies) or anticipation of the need for major surgery during study
             treatment

          -  Known history of autoimmune disease as defined in the protocol

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

          -  Participants with bilateral lung lesions and dyspnea and/or oxygen saturation level
             (SaO2) less than 92% (at rest, room air and exertion) or participants with lobectomy
             or pneumonectomy with lung metastases in the remaining lung and either dyspnea or SaO2
             less than 92% (at rest, room air and exertion) at baseline

          -  Pregnant or breast-feeding

          -  Known hypersensitivity to any of the components of RO6958688 and atezolizumab;
             hypersensitivity to Chinese hamster ovary cell products or other recombinant human
             antibodies

          -  Investigational therapy (defined as treatment for which there is no regulatory
             authority approved indication) or last dose of prior immunotherapies within 28 days
             prior to Cycle 1 Day 1. Participants previously treated with anti-programmed
             death-ligand 1 (PD-L1), or anti-PD-1 are excluded

          -  Last dose of any approved anti-cancer therapy within 28 days prior to the first
             RO6958688 infusion

          -  Prior systemic corticosteroids greater than 10mg prednisone (or equivalent) within 14
             days of Cycle 1 Day 1. Inhaled and/or topical steroids are permitted

          -  Expected need for regular immunosuppressive therapy

          -  Radiotherapy within the last 28 days before Cycle 1 Day 1 with the exception of
             limited-field palliative radiotherapy