Inclusion Criteria:

          -  Capable of providing signed informed consent, which includes compliance with the
             requirements and restrictions listed in the ICF and in the Clinical Study Protocol
             (CSP).

          -  Provision of signed and dated, written ICF prior to any mandatory study specific
             procedures, sampling, and analyses.

          -  Female ≥18 years of age at the time of signing the ICF.

          -  Eastern Cooperative Oncology Group performance status 0 to 1 within 28 days of
             randomization.

          -  Participants with relapsed histologically confirmed diagnosis of high grade epithelial
             ovarian cancer (including primary peritoneal and/or fallopian tube cancer), with
             disease relapse on or after completion of PARPi maintenance therapy and who have not
             received any intervening systemic treatment since discontinuation of PARPi (this
             excludes the platinum based chemotherapy received during Screening Part 1 of this
             study).

          -  A minimum of 6 months of prior PARPi treatment received in the maintenance setting for
             PSR ovarian cancer (a minimum of 12 months is required if the participant received
             PARPi maintenance following first line chemotherapy). If the prior PARPi used was
             olaparib then participants must have received treatment without significant toxicity
             or the need for a permanent dose reduction.

          -  Disease relapse in the second line (first relapse) or third line (second relapse)
             setting.

          -  Able to provide and consent to the collection of a contemporaneous tumor tissue biopsy
             and blood sample.

          -  Able to provide a Formalin Fixed Paraffin Embedded archival tumour tissue block from
             the time of primary tumour diagnosis (taken ideally prior to receiving any systemic
             treatment, and definitely prior to first PARPi treatment) for prospective Breast
             cancer susceptibility gene (BRCA) status testing. If tumour blocks are unavailable,
             tissue sections are acceptable with a minimum requirement of at least 20 unstained
             sections on uncharged slides without cover slips. Fine needle aspirates are not
             acceptable.

          -  Where the patient has previously been tested for germline or somatic BRCA alterations
             using a verified and well-validated test in line with local regulations, performed in
             a locally accredited laboratory (eg, College of American Pathologists/Clinical
             Laboratory Improvement Amendments laboratory, where available), and signed consent to
             provide a copy of the BRCA report.

          -  Platinum-sensitive disease at the time of disease relapse, i.e, platinum-treatment
             free survival of greater than 6 months as defined by the Gynecological Cancer
             Intergroup (Wilson et al 2017).

          -  For the platinum-based chemotherapy course received following pre screening (Part 1)
             and prior to entering the main screening (Part 2).

          -  Any prior palliative radiation must have been completed at least 7 days prior to the
             start of study drugs, and participants must have recovered from any acute adverse
             effects prior to the start of study treatment.

          -  Normal organ and bone marrow function measured within 28 days prior to randomization.

          -  Participant is willing and able to comply with the CSP for the duration of the study
             including undergoing treatment and scheduled visits and examinations.

          -  Participants must have a life expectancy of ≥16 weeks.

          -  Participants must be able to swallow tablets whole.

          -  For inclusion in the optional (deoxyribonucleic acid) genetics research, study
             participants must fulfil the following criterion:

        Provide informed consent for the genetic sampling and analyses. If a participant declines
        to participate in the genetics research, there will be no penalty or loss of benefit to the
        participant. A participant who declines genetics research participation will not be
        excluded from any other aspect of the main study.

          -  Participant's body weight must be >30 kg.

          -  Postmenopausal or evidence of non-childbearing status for women of childbearing
             potential.

          -  Women of childbearing potential and their partners, who are sexually active, must
             agree to the use of 2 highly effective forms of contraception in combination from the
             signing of the informed consent (Screening Part 1), throughout the period of taking
             study treatment and for at least 6 months after the last dose of study drug(s).

        Exclusion Criteria:

          -  Participants who have had drainage of their ascites during the final 2 cycles of their
             last chemotherapy regimen or during the period between completion of chemotherapy and
             first dose of study treatment.

          -  Participants with current signs or symptoms of bowel obstruction, including
             sub-occlusive disease, related to underlying disease.

          -  History of leptomeningeal carcinomatosis.

          -  Participants with symptomatic uncontrolled brain metastases.

               1. A scan to confirm the absence of brain metastases is not required.

               2. Participants whose brain metastases have been treated may participate provided
                  they show radiographic stability. In addition, any neurologic symptoms that
                  developed either as a result of the brain metastases or their treatment must have
                  resolved or be stable (Common Terminology Criteria for Adverse Events [CTCAE]
                  Grade <2) either, without the use of steroids, or are stable on a steroid dose of
                  ≤10 mg/day of prednisone or its equivalent and stable on anti convulsants if
                  required for at least 14 days prior to the start of treatment.

               3. Participants with spinal cord compression are not eligible unless considered to
                  have received definitive treatment for this and have evidence of clinically SD
                  for >28 days and have not received steroid treatment for at least 14 days prior
                  to the start of study treatment.

          -  History of another primary malignancy except for:

               1. Malignancy treated with curative intent and with no known active disease ≥5 years
                  before the first dose of IP and of low potential risk for recurrence;

               2. Adequately treated non-melanoma skin cancer or lentigo maligna without evidence
                  of disease;

               3. Adequately treated carcinoma in situ without evidence of disease.

          -  Major surgical procedures (as defined by the investigator) ≤28 days of beginning study
             treatment, or minor surgical procedures ≤7 days. No waiting period required following
             port a cath or other central venous access placement.

          -  Persistent toxicities (≥CTCAE Grade 2) caused by previous cancer therapy, excluding
             alopecia and CTCAE Grade 2 peripheral neuropathy. Note: participants with signs of
             ongoing complications from radiation therapy are not eligible for this study.

          -  Participants with myelodysplastic syndrome (MDS) /acute myeloid leukemia (AML) or with
             features suggestive of MDS/AML.

          -  Resting electrocardiogram indicating uncontrolled, potentially reversible cardiac
             conditions, as judged by the investigator, or participants with congenital long QT
             syndrome.

          -  Known allergy or hypersensitivity to any of the study drugs or any of the study drug
             excipients.

          -  History of allogeneic organ transplantation including previous allogeneic bone marrow
             transplant or double umbilical cord blood transplantation.

          -  History of active primary immunodeficiency.

          -  Active infection including tuberculosis (TB), hepatitis B (known positive hepatitis B
             virus [HBV] surface antigen [HBsAg] result), hepatitis C (HCV), or human
             immunodeficiency virus (HIV) (positive HIV 1/2 antibodies). Participants with a past
             or resolved HBV infection are eligible. Participants positive for HCV antibody are
             eligible only if polymerase chain reaction is negative for HCV ribonucleic acid.

          -  Uncontrolled intercurrent illness, including but not limited to, ongoing or active
             infection, symptomatic congestive heart failure, uncontrolled hypertension, unstable
             angina pectoris, uncontrolled cardiac arrhythmia, active interstitial lung disease,
             serious chronic gastrointestinal conditions associated with diarrhea, or psychiatric
             illness/social situations.

          -  Current dependency on total parenteral nutrition or intravenous (iv) fluid hydration.

          -  Whole blood transfusions in the last 120 days prior to entry to the study.

          -  Cytotoxic chemotherapy, hormonal or non hormonal targeted therapy within 21 days of
             Cycle 1, Day 1 is not permitted. The participant can receive a stable dose of
             bisphosphonates or denosumab for bone metastases, before and during the study as long
             as these were started at least 5 days prior to study treatment.

          -  Participation in another clinical study with an IP during the chemotherapy course
             immediately prior to randomisation or during the course of the study.

          -  Previous treatment with ceralasertib or other ataxia telangiectasia and Rad3-related
             protein, Checkpoint kinase 1 or deoxyribonucleic acid damage response inhibitor
             (excluding PARPi).

          -  Exposure to a small molecule IP within 30 days or 5 half-lives (whichever is longer)
             prior to randomization. The minimum washout period for immunotherapy and bevacizumab
             shall be 42 days.

          -  Receipt of live attenuated vaccine within 30 days prior to the first dose of study
             treatment.

          -  Concomitant use of known strong cytochrome P450 (CYP) 3A inhibitors or moderate CYP3A
             inhibitors. The required washout period prior to starting study treatment is 2 weeks.

          -  Concomitant use of known strong CYP3A inducers or moderate CYP3A inducers. The
             required washout period prior to starting study treatment is 5 weeks for enzalutamide
             or phenobarbital and 3 weeks for other agents.

          -  Involvement in the planning and/or conduct of the study (applies to Sponsor staff
             and/or staff at the study site).

          -  Previous randomization in the present study.

          -  Judgment by the investigator that the participant should not participate in the study
             if the participant is unlikely to comply with study procedures, restrictions and
             requirements.

          -  Pregnant or lactating women.