Inclusion Criteria:

          -  Radiologically confirmed (ie, CT scan and/or MRI) persistent or recurrent endometrial
             cancer

          -  Patients must have one of the following pathologically documented, definitively
             diagnosed tumor types: Uterine serous carcinoma (Pure or Mixed), Grade 2 or Grade 3
             endometrial adenocarcinoma, or Carcinosarcoma with high grade serous or Grade 2/3
             endometrioid components

          -  Have measurable disease

          -  FRα-positive tumor expression as defined in the protocol

          -  Have at least one "target lesion" to be used to assess response as defined by RECIST
             v1.1

          -  Patients must have received prior treatment with ≤ 3 prior lines of therapy for
             recurrent disease; hormonal agents are not considered a line of therapy; prior
             treatment with folate receptor-targeting investigational agents is not allowed

          -  Have Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1

          -  Time from prior therapy: Systemic anti-neoplastic therapy: five half-lives or four
             weeks, whichever is shorter; Radiotherapy: wide-field radiotherapy completed at least
             four weeks, or focal radiation completed at least two weeks, prior to starting study
             treatment

          -  Patients must have resolution of toxic effect(s) of the most recent prior chemotherapy

          -  Patients must have adequate hematologic, liver and kidney function as defined in the
             protocol

          -  Women of child bearing potential (WCBP), must agree to use effective contraceptive
             methods during study treatment and for at least twelve weeks after the last dose of
             IMGN853

          -  WCBP must have a negative pregnancy test within 4 days prior to the first dose of
             study treatment

          -  At time of initial surgery, patient may have either been optimally or suboptimally
             debulked

          -  Have signed the informed consent form, and willing to adhere to the study visit
             schedule and other protocol requirements

          -  ≥ 18 years of age

        Exclusion Criteria:

          -  Active or chronic corneal disorder

          -  Serious concurrent illness or clinically-relevant active infection as defined in the
             protocol

          -  Clinically-significant cardiac disease such as recent myocardial infarction (≤ 6
             months prior to day 1), unstable angina pectoris, uncontrolled congestive heart
             failure, uncontrolled hypertension, prior history of hypertensive crisis or
             hypertensive encephalopathy, uncontrolled cardiac arrhythmias, clinically-significant
             vascular disease, severe aortic stenosis, clinically significant peripheral vascular
             disease, or cardiac toxicity following prior chemotherapy

          -  History of neurological conditions

          -  History of hemorrhagic or ischemic stroke within the last 6 months

          -  History of cirrhotic liver disease

          -  Previous clinical diagnosis of non-infectious pneumonitis

          -  Prior hypersensitivity to monoclonal antibodies

          -  Women who are pregnant or breast feeding

          -  Carcinomatous meningitis, untreated central nervous system (CNS) disease or
             symptomatic CNS metastasis

          -  History or evidence of thrombotic or hemorrhagic disorders within 6 months before
             first study treatment

          -  Required used of folate-containing supplements (e.g. folate deficiency)

          -  Has a known additional malignancy that is progressing or required active treatment
             within 3 years of first dose of study treatment