A Study of TAK-861 in Participants With Narcolepsy Type 2

Inclusion Criteria: \- The participant is aged 18 to 70 years, inclusive, at the time of signing the informed consent form (ICF). Note: In Japan, participants aged 16 to 70 years, inclusive, may be included. \- The participant has an International Classification of Sleep Disorders, 3rd edition (ICSD-3) diagnosis of NT2 by preceding polysomnography (PSG)/ multiple sleep latency test (MSLT), performed within the past 5 years. Note: If there is a potential participant with NT2 for whom a diagnostic nocturnal polysomnography (nPSG)/MSLT was performed more than 5 years ago or is not available, the site may repeat the diagnostic PSG/MSLT. Exclusion Criteria: * The participant has a current medical disorder, other than narcolepsy without cataplexy, associated with EDS. * The participant has history of epilepsy, seizure, or convulsion, or has a family history of inherited disorders associated with seizure (except for a single febrile seizure in childhood). * The participant has one or more of the following psychiatric disorders: 1. Any current unstable psychiatric disorder. 2. Current or history of manic or hypomanic episode, schizophrenia or any other psychotic disorder, including schizoaffective disorder, major depression with psychotic features, bipolar depression with psychotic features, obsessive compulsive disorder, mental retardation, organic mental disorders, or mental disorders due to a general medical condition as defined in the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5). 3. Current diagnosis or history of substance use disorder as defined in the DSM-5. Note: If the history of substance use disorder is more than 12 months before baseline, the participant may be allowed to enroll in the study after consultation with the sponsor or designee. (Participant must also have negative urine drug screen at the screening and Day -2 visit.) 4. Current active major depressive episode (MDE) or who have had an active MDE in the past 6 months. * The participant has a history of cerebral ischemia, transient ischemic attack (\<5 years ago), intracranial aneurysm, or arteriovenous malformation. * The participant had major surgery or donated or lost 1 unit of blood (approximately 500 mL) within 4 weeks before the screening visit.