A Study of TAK-861 for the Treatment of Narcolepsy Type 1

The main aim of this study is to learn how effective TAK-861 (oveporexton) is in improving excessive sleepiness during the day (called excessive daytime sleepiness or EDS) after 3 months of treatment. Other aims are to learn how effective TAK-861 (oveporexton) is in lowering the number of sudden, unexpected attacks of muscle weakness while staying conscious (cataplexy) in a week; to learn the effect TAK-861 (oveporexton) has on participants' ability to maintain attention, participant's overall quality of life, the spectrum of narcolepsy symptoms, and daily life functions; and to learn about the safety of TAK-861 (oveporexton).

Inclusion Criteria: 1. The participant has a body mass index (BMI) within the range 18 to 40 kilograms per meter square (kg/m\^2). 2. The participant has an International Classification of Sleep Disorders, Third Edition (ICSD-3) or International Classification of Sleep Disorders, Third Edition, Text Revision (ICSD-3-TR) diagnosis of NT1. 3. The participant has greater than or equal to (≥)4 partial or complete episodes of cataplexy/week (WCR). 4. The participant is positive for the human leukocyte antigen (HLA) genotype HLA-DQB1\*06:02 or results from radioimmunoassay indicate the participant's cerebrospinal fluid (CSF) orexin (OX)/hypocretin-1 concentration is less than or equal to (≤)110 picograms per milliliter (pg/mL) \[or less than one-third of the mean values obtained in normal participants within the same standardized assay\]. Exclusion Criteria: 1. The participant has a current medical disorder, other than narcolepsy with cataplexy, associated with EDS. 2. The participant: (a) has a history of myocardial infarction; (b) has a history of clinically significant hepatic disease, thyroid disease, coronary artery disease, cardiac rhythm abnormality or heart failure; or (c) has any medical condition (such as unstable cardiovascular, pulmonary, renal or gastrointestinal disease). 3. The participant has current or recent (within 6 months) gastrointestinal disease that is expected to influence the absorption of drugs. 4. The participant has a history of cancer in the past 5 years. 5. The participant has a clinically significant history of head injury or head trauma. 6. The participant has a history of epilepsy, seizure, or convulsion. 7. The participant has a history of cerebral ischemia, transient ischemic attack (\