Inclusion Criteria:

          -  Documented informed consent of the participant and/or legally authorized
             representative

               -  Assent, when appropriate, will be obtained per institutional guidelines

          -  Agreement to allow the use of archival tissue from diagnostic tumor biopsies

               -  If unavailable, exceptions may be granted with study principal investigator (PI)
                  approval

          -  Eastern Cooperative Oncology Group (ECOG) =< 2

          -  Histologically confirmed diagnosis of

               -  Cohort A. Acute lymphoblastic leukemia

                    -  B-cell phenotype: patients with relapsed or refractory ALL who have received
                       at least 2 prior regimens and failed or are ineligible for CD19-based
                       targeted therapy

                    -  T-cell phenotype: patients with relapsed or refractory who have received at
                       least 1 prior regimen

               -  Cohort B. Other CD123+ hematological malignancies that failed standard regimens,
                  excluding acute myeloid leukemia and myelodysplastic syndrome

                    -  Blastic plasmacytoid dendritic cell neoplasm (BPDCN) patients who have
                       failed or relapsed after initial therapy

                    -  Chronic myelocytic leukemia (CML) patients who have failed or relapsed or
                       ineligible for third generation tyrosine kinase inhibitor (ponatinib)

                    -  Hairy cell leukemia patients who have failed or progressed shortly after
                       purine analogs or failed 2 cycles of purine analog

                    -  Systemic mastocytosis patients who have failed or progressed on midostaurin

                    -  Hodgkin lymphoma patients who have failed or relapsed after PD-1/PD-L1-
                       inhibitors and brentuximab vedotin

                    -  Advanced acute leukemia patients with ambiguous lineage or biphenotypic
                       leukemia that failed 2 lines of prior regimens

                    -  Patients with any other advanced CD123+ hematological malignancy who have
                       failed standard therapy per the treating physician's judgement

          -  Relapsed or refractory disease as defined above

          -  Tumor expressing CD123 either by flow cytometry or immunohistochemistry staining

          -  Measurable disease of at least 1.5 cm on computed tomography (CT)/magnetic resonance
             imaging (MRI) for cases without bone marrow involvement

          -  Peripheral blast count < 20,000/ul at the time of initiation of infusion on Cycle 1
             Day 1

          -  Life expectancy of at least 4 weeks

          -  Fully recovered from the acute toxic effects (except alopecia) to =< grade 1 to prior
             anti-cancer therapy

          -  Absolute neutrophil count (ANC) >= 1000/ul (without bone marrow involvement, performed
             within 14 days prior to day 1 of protocol therapy)

          -  Platelets >= 75,000/ul (without bone marrow involvement, performed within 14 days
             prior to day 1 of protocol therapy)

          -  Lumbar puncture to assess presence of central nervous system (CNS) disease if there
             are symptoms and signs concerning for CNS involvement (performed within 14 days prior
             to day 1 of protocol therapy)

          -  Total bilirubin =< 1.5 X upper limit of normal (ULN) (unless has Gilbert's disease)
             (performed within 14 days prior to day 1 of protocol therapy)

          -  Aspartate aminotransferase (AST) =< 2.5 x ULN (performed within 14 days prior to day 1
             of protocol therapy)

          -  Alanine aminotransferase (ALT) =< 2.5 x ULN (performed within 14 days prior to day 1
             of protocol therapy)

          -  Left ventricular ejection fraction (LVEF) >= 50%. Note: To be performed within 28 days
             prior to day 1 of protocol therapy

          -  Corrected QT (QTc) =< 480 ms. Note: To be performed within 28 days prior to day 1 of
             protocol therapy

          -  If able to perform pulmonary function tests: forced expiratory volume in 1 second
             (FEV1), forced vital capacity (FVC) and DLCO (diffusion capacity) >= 50% of predicted
             (corrected for hemoglobin). If unable to perform pulmonary function tests: oxygen (O2)
             saturation > 90% on room air. Note To be performed within 28 days prior to day 1 of
             protocol therapy

          -  Calculated or measured creatinine clearance of > 50 ml/min (performed within 14 days
             prior to Day 1 of protocol therapy)

          -  Women of childbearing potential (WOCBP): negative urine or serum pregnancy test. If
             the urine test is positive or cannot be confirmed as negative, a serum pregnancy test
             will be required (performed within 14 days prior to day 1 of protocol therapy)

          -  Agreement by females and males of childbearing potential* to use an effective method
             of birth control or abstain from heterosexual activity for the course of the study
             through at least 6 months after the last dose of protocol therapy

               -  Childbearing potential defined as not being surgically sterilized (men and women)
                  or have not been free from menses for > 1 year (women only)

        Exclusion Criteria:

          -  Autologous or allogeneic hematopoietic cell transplant performed within 100 days prior
             to study drug administration in Day 1 of Cycle 1 of protocol therapy

               -  However, patients who received allogeneic hematopoietic cell transplantation
                  (HCT) more than 100 days are allowed if no active graft versus host disease
                  (GVHD) > grade 1, not actively on systemic immunosuppressive therapy and off
                  calcineurin inhibitors for at least 4 weeks prior to start therapy

          -  Chemotherapy, radiation therapy, biological therapy, within 14 days prior to Day 1 of
             protocol therapy. Maintenance-type ALL chemotherapies, including vincristine and
             mercaptopurine are allowed up to 7 days before starting therapy. High dose steroids
             are allowed up to 3 days before starting therapy. Cytoreduction with hydroxyurea is
             allowed to control leukocytosis until to the day of starting therapy. Hydroxyurea can
             be given during cycle 1 of flotetuzumab administration to control leukocytosis but
             need to be discussed with the study PI

          -  Previous treatment with immunotherapeutic agents (for example chimeric antigen
             receptor [CAR] T cells, long acting bispecific antibodies, etc) in the 28 days period
             prior to study drug administration on Day 1 Cycle 1, with the exception of short-half
             bispecific antibodies (blinatumomab) where the washout period is only 14 days

          -  Requirement, at the time of study entry, for concurrent steroid > 10 mg/day of oral
             prednisone or the equivalent, except steroid inhaler, nasal spray or ophthalmic
             solution

          -  Use of immunosuppressant medications (other than steroid as noted above) in the 2
             weeks prior to study drug administration (Cycle 1 Day 1)

          -  Known central nervous system involvement. Patients with suspected CNS involvement must
             be evaluated by lumbar puncture and be free of CNS disease prior to study entry.
             Previously treated CNS involvement is allowed provided adequate treatment has been
             provided and the patient is free of CNS disease

          -  History of allergic reactions attributed to compounds of similar chemical or biologic
             composition to flotetuzumab

          -  Any active untreated autoimmune disorders (with the exception of vitiligo)

          -  Dementia or altered mental status that would preclude sufficient understanding to
             provide informed consent

          -  Second primary malignancy that requires active therapy. Adjuvant hormonal therapy is
             allowed

          -  Active uncontrolled infection

          -  Significant pulmonary compromise

          -  Unstable angina or clinically significant heart disease (left ventricular ejection
             fraction < 50%)

          -  Major trauma or surgery within 4 weeks before enrollment

          -  Clinically significant uncontrolled illness

          -  Females only: Pregnant or breastfeeding

          -  Any other condition that would, in the Investigator's judgment, contraindicate the
             patient's participation in the clinical study due to safety concerns with clinical
             study procedures

          -  Prospective participants who, in the opinion of the investigator, may not be able to
             comply with all study procedures (including compliance issues related to
             feasibility/logistics)