Inclusion Criteria:

   - Newly-diagnosed brain cancer with histopathological diagnosis of GBM;

   - Karnofsky Performance Status (KPS) rating of >/=70 at baseline;

   - Receive dexamethasone equivalent dose /=
   three days prior to Day 0;

   - Recovery from the effects of prior GBM surgery as defined by the Investigator;

   - Electrocardiogram (ECG) with no clinically significant findings as assessed by the
   Investigator;

   - Adequate organ function as demonstrated by hematological, renal, hepatic laboratory
   assessments;

   - Agree that during the trial, men will not father a child, and women cannot be or
   become pregnant. Participants must be of non-child bearing potential or agree to use
   one highly effective or combined contraceptive methods that result in a failure rate
   of <1% per year during the treatment period and at least through week 12 after last
   dose;

   - Ability to tolerate magnetic resonance imaging (MRI).

Exclusion Criteria:

   - Presence of greater than 1 cm x 1 cm residual tumor enhancement on postoperative MRI;

   - Multifocal disease or leptomeningeal disease (LM) disease on post-operative MRI;

   - Not scheduled to start radiation within 42 days of surgical resection of tumor;

   - Dexamethasone equivalent dose >2 mg per day;

   - Prior treatment with an agent that blocks the PD-1/PD-Ligand 1 pathway;

   - Receipt of previous approved or investigative immune modulatory agent within 28 days
   of receiving the first dose of treatment;

   - Prior treatment with idelalisib;

   - Past, current or planned treatment with tumor treatment fields; oncolytic viral
   treatment; or prior exposure to an investigational agent or device within 28 days of
   receiving the first dose of treatment;

   - Allergy or hypersensitivity to cemiplimab or to any of its excipients;

   - History of documented allergic reactions or acute hypersensitivity reaction attributed
   to antibody treatments;

   - Ongoing or recent (within 5 years) evidence of autoimmune disease that required
   treatment with systemic immunosuppressive treatments;

   - Diagnosis of immunodeficiency or treatment with systemic immunosuppressive therapy
   within 28 days prior to the first dose of trial treatment, other than dexamethasone
   for the underlying disease under investigation, as noted in the inclusion criteria;

   - History of clinically significant, medically unstable disease which, in the judgment
   of the investigator, would jeopardize the safety of the subject, interfere with trial
   assessments or endpoint evaluation, or otherwise impact the validity of the trial
   results.