Inclusion Criteria:  Participant has provided informed consent/assent prior to initiation of any study specific activities/procedures. Male and female participants ≥ 18 years of age (or legal adult age within country) at the time of signing the informed consent. Histologically or cytologically confirmed relapsed/refractory SCLC Participants who progressed or recurred following 1 platinum-based regimen and at least 1 other prior line of therapy Eastern Cooperative Oncology Group (ECOG) performance status of 0 1. Minimum life expectancy of 12 weeks. Measurable lesions as defined per RECIST 1.1 within 21 days prior to the first dose of AMG 757. Participants with treated brain metastases are eligible provided they meet defined criteria.  Exclusion Criteria:  Disease Related  Untreated or symptomatic brain metastases and leptomeningeal disease. Has evidence of interstitial lung disease or active, non-infectious pneumonitis. Participants who experienced recurrent pneumonitis (grade 2 or higher) or severe, life-threatening immune-mediated adverse events or infusion-related reactions including those that lead to permanent discontinuation while on treatment with immuno-oncology agents. Unresolved toxicity from prior anti-tumor therapy, defined as per protocol.  Other Medical Conditions  History of other malignancy within the past 2 years, with exceptions Myocardial infarction and/or symptomatic congestive heart failure (New York Heart Association > class II) within 12 months of first dose of AMG 757. History of arterial thrombosis (eg, stroke or transient ischemic attack) within 12 months of first dose of AMG 757. Presence of fungal, bacterial, viral, or other infection requiring oral or IV antimicrobials for management within 7 days of first dose AMG 757. Presence of any indwelling line or drain. History of hypophysitis or pituitary dysfunction. Exclusion of hepatitis infection based on the results and/or criteria per protocol. Major surgery within 28 days of first dose AMG 757. History or evidence of severe acute respiratory syndrome coronavirus 2 (SARS CoV-2) infection unless agreed upon with medical monitor and meeting the following criterion: no acute symptoms of coronavirus disease 2019 (COVID 19) within 14 days prior to first dose of AMG 757 (counted from day of positive test for asymptomatic participants)  Prior/Concomitant Therapy  Participant received prior therapy with AMG 757. Prior anti-cancer therapy within 28 days prior to first dose of AMG 757. Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of AMG 757. Live and live-attenuated vaccines within 4 weeks prior to the start off AMG 757 treatment, during treatment, and until end of last study dose.  Other Exclusions  Female participants of childbearing potential unwilling to use protocol specified method of contraception during treatment and for an additional 42 days after the last dose of AMG 757. Female participants who are breastfeeding or who plan to breastfeed while on study through 42 days after the last dose of AMG 757. Female participants planning to become pregnant while on study through 42 days after the last dose of AMG 757. Female participants of childbearing potential with a positive pregnancy test assessed at screening and/or day 1 by a highly sensitive urine or serum pregnancy test. Male participants with a female partner of childbearing potential who are unwilling to practice sexual abstinence (refrain from heterosexual intercourse) or use contraception during treatment and for an additional 42 days after the last dose of AMG 757. Male participants with a pregnant partner who are unwilling to practice abstinence or use a condom during treatment and for an additional 42 days after the last dose of AMG 757. Male participants unwilling to abstain from donating sperm during treatment and for an additional 42 days after the last dose of AMG 757. Participant has known sensitivity to any of the products or components to be administered during dosing. Participant likely to not be available to complete all protocol-required study visits or procedures, and/or to comply with all required study procedures. History or evidence of any other clinically significant disorder, condition or disease determined by the investigator or Amgen physician.