Pembrolizumab/Placebo Plus Paclitaxel With or Without Bevacizumab for Platinum-resistant Recurrent Ovarian Cancer (MK-3475-B96/KEYNOTE-B96/ENGOT-ov65).

Inclusion Criteria:

* Has histologically confirmed epithelial ovarian, fallopian tube, or primary peritoneal carcinoma.
* Has received 1 or 2 prior lines of systemic therapy for ovarian cancer (OC), including at least 1 prior platinum-based therapy. Participants may have received a prior poly (ADP-ribose) polymerase inhibitor (PARPi), anti-programmed cell death 1 protein (PD-1)/anti-programmed cell death ligand 1 (PD-L1) therapy, bevacizumab, or hormonal therapy; these will not be considered a separate line of therapy. Any chemotherapy regimen change due to toxicity in the absence of disease progression will be considered part of the same line of therapy.
* Has provided documented informed consent for the study.
* Has radiographic evidence of disease progression within 6 months (180 days) after the last dose of platinum-based chemotherapy for OC (i.e., platinum-resistant disease).
* Is a candidate for paclitaxel chemotherapy (and bevacizumab, if using).
* Has an Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 1 assessed within 3 days before randomization.
* For a female participant, she is not pregnant or breastfeeding, and at least one of the following conditions applies: Is not a woman of childbearing potential (WOCBP) OR Is a WOCBP and uses a contraceptive method that is highly effective (with a failure rate of \<1% per year).
* Has radiographically evaluable disease, either measurable or nonmeasurable per Response Evaluation Criteria In Solid Tumors (RECIST) 1.1, as assessed by the local site investigator.
* Archival tumor tissue sample or newly obtained core or incisional/excisional biopsy of a tumor lesion not previously irradiated has been provided.
* Have adequate organ function.

Exclusion Criteria:

* Has nonepithelial cancers, borderline tumors, mucinous, seromucinous that is predominantly mucinous, malignant Brenner's tumor and undifferentiated carcinoma.
* Has primary platinum-refractory disease, defined as disease that has progressed per radiographic imaging while receiving or within 28 days of the last dose of first-line platinum-based therapy.
* Has prior disease progression on weekly paclitaxel alone.
* Has received \>2 prior lines of systemic therapy for OC.
* Has received prior systemic anticancer therapy including investigational agents or maintenance therapy (including bevacizumab maintenance therapy), within 4 weeks before randomization.
* Has received prior radiation therapy within 2 weeks of start of study intervention.
* Has not recovered adequately from surgery and/or any complications from the surgery.
* Has received colony-stimulating factors (e.g., granulocyte colony-stimulating factor \[G-CSF\], granulocyte-macrophage colony-stimulating factor,\[GM-CSF\] or recombinant erythropoietin) within 4 weeks before randomization.
* Has received a live or live-attenuated vaccine within 30 days before the first dose of study intervention.
* Has received investigational agent or has used an investigational device within 4 weeks prior to study intervention.
* Has a diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy.
* Has a known additional malignancy that is progressing or has required active treatment within the past 3 years.
* Has known active central nervous system (CNS) metastases and/or carcinomatous meningitis.
* Has severe hypersensitivity (≥Grade 3) to pembrolizumab, paclitaxel, or bevacizumab (if using) and/or any of their excipients.
* Has an active autoimmune disease that has required systemic treatment in the past 2 years.
* Has a history of (noninfectious) pneumonitis/interstitial lung disease that required steroids or has current pneumonitis/interstitial lung disease.
* Has an active infection requiring systemic therapy.
* Has a known history of human immunodeficiency virus (HIV) infection.
* Has a known history of Hepatitis B or known active Hepatitis C virus infection.
* Has a history or current evidence of any condition, therapy, laboratory abnormality, or other circumstance that might confound the results of the study.
* Has a known psychiatric or substance abuse disorder that would interfere with the participant's ability to cooperate with the requirements of the study.
* Participant, in the judgement of the investigator, is unlikely to comply with the study procedures, restrictions, and requirements of the study.
* Has had an allogenic tissue/solid organ transplant.

For bevacizumab treatment

* Has uncontrolled hypertension.
* Has current, clinically relevant bowel obstruction including related to underlying epithelial OC, abdominal fistula or gastrointestinal perforation, intra-abdominal abscess, or evidence of rectosigmoid involvement by pelvic exam.
* Has a history of thrombotic disorders, hemorrhage, hemoptysis, or active gastrointestinal bleeding within 6 months before randomization.