Inclusion Criteria:  Participant has provided informed consent/assent prior to initiation of any study specific activities/procedures. Age ≥18 years. Pathologically documented metastatic colorectal adenocarcinoma with Kirsten rat sarcoma (KRAS) p.G12C mutation as determined by central testing. Participants will have received at least 1 prior line of therapy for metastatic disease. Participants must have received and progressed or experienced disease recurrence on or after fluoropyrimidine, irinotecan, and oxaliplatin given for metastatic disease unless the participant, in the opinion of the investigator, is not a candidate for fluoropyrimidine, irinotecan, or oxaliplatin, in which case, the participant may be eligible after investigator discussion with Amgen medical monitor provided participant has received at least one prior line of therapy for metastatic disease and provided trifluridine and tipiracil or regorafenib is deemed the appropriate next line of therapy for the participant. Measurable disease per Response Evaluation Criteria In Solid Tumors (RECIST) 1.1 criteria. Lesions previously radiated are not considered measurable unless they have progressed after radiation. Eastern Cooperative Oncology Group (ECOG) Performance Status of ≤2. Life expectancy of >3 months, in the opinion of the investigator.  Adequate hematologic and end-organ function, defined as the following within 10 days prior to randomization:  Absolute neutrophil count (ANC) ≥1.5 x 10^9/L (without granulocyte colony stimulating factor support within 2 weeks of laboratory test used to determine eligibility). Hemoglobin ≥9.0 g/dL (without transfusion within 2 weeks of laboratory test used to determine eligibility). Platelet count ≥100 x 10^9/L (without transfusion within 2 weeks of laboratory test used to determine eligibility). Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤2.5 times the upper limit of normal (ULN). Serum bilirubin ≤1.0 x ULN. For participants with Gilbert's disease, direct bilirubin ≤1.0 x ULN. International normalized ratio (INR) and activated partial thromboplastin time (or partial thromboplastin time) ≤1.5 x ULN. Prothrombin time (PT) ≤1.5 x ULN may be used instead of INR for sites whose labs do not report INR. Estimated glomerular filtration rate based on Modification of Diet in Renal Disease (MDRD) calculation ≥30 mL/min/1.73 m^2. Fridericia's Correction Formula (QTcF) ≤470 msec.  Exclusion Criteria:  Active brain metastases. Participants who have had brain metastases resected or have received radiation therapy ending at least 4 weeks prior to study day 1 are eligible if they meet all of the following criteria: a) residual neurological symptoms grade ≤2; b) on stable doses of dexamethasone or equivalent for at least 2 weeks, if applicable; and c) follow-up magnetic resonance imaging (MRI) performed within 28 days of day 1 shows no progression or new lesions appearing. History or presence of hematological malignancies unless curatively treated with no evidence of disease ≥2 years.  History of other malignancy within the past 3 years, with the following exceptions:  Malignancy treated with curative intent and with no known active disease present for ≥3 years before enrollment and felt to be at low risk for recurrence by the treating physician. Adequately treated non-melanoma skin cancer or lentigo maligna without evidence of disease. Adequately treated cervical carcinoma in situ without evidence of disease. Adequately treated breast ductal carcinoma in situ without evidence of disease. Prostatic intraepithelial neoplasia without evidence of prostate cancer. Adequately treated urothelial papillary non-invasive carcinoma or carcinoma in situ. Leptomeningeal disease. Significant gastrointestinal (GI) disorder that results in significant malabsorption, requirement for intravenous (IV) alimentation, or inability to take oral medication. History of interstitial pneumonitis or pulmonary fibrosis or evidence of interstitial pneumonitis or pulmonary fibrosis. Significant cardiovascular disease, such as New York Heart Association cardiac disease (Class II or greater), myocardial infarction within 6 months prior to randomization, unstable arrhythmias or unstable angina. Previous treatment with a KRAS G12C inhibitor.