This trial will evaluate the effects of Lu AF28996 in adult Japanese men and women with Parkinson's disease (PD). The main goals of the trial are to learn more about a) the safety and tolerability of Lu AF28996 (any new or worsening medical issues the participants have with treatment), and b) the pharmacokinetic parameters of Lu AF28996 (how the drug is absorbed, distributed, and processed by the body). Participants will take Lu AF28996 capsules orally once a day (OD). The participants will start with a low dose of Lu AF28996, which will be increased gradually over a period of 2 weeks, and then decreased gradually over a period of about 2 weeks.

Inclusion Criteria:

• The participant is diagnosed with idiopathic PD (consistent with the United Kingdom Parkinson's Disease Society Brain Bank Criteria for the Diagnosis of Parkinson's Disease) and should not have more than 1 first-degree relative with PD.
• The participant's Modified Hoehn and Yahr score is ≤3 in the ON state and ≤4 in the OFF state.
• The participant experiences well-recognizable and predictable motor fluctuations (≥1.5 hours of OFF-periods in the awake time, predictable morning OFF episodes included) causing clinically significant disability during the 7-week Screening Period.
• The participant currently has a good response to levodopa and has been receiving a stable dose of levodopa (≥3 doses per day of levodopa/dopa decarboxylase inhibitor therapy) ≥4 weeks prior to screening.
• The participant is Japanese, defined as having four Japanese grandparents and being born in Japan.

Exclusion Criteria:

• The participant has or had one or more of the following conditions that are considered clinically relevant in the context of the study; other neurological disorder, psychiatric disorder, seizure disorder or encephalopathy, respiratory disease, hepatic impairment or renal insufficiency, metabolic disorder, endocrinological disorder, hematological disorder, infectious disorder, any clinically significant immunological condition, or a history of narrow-angle glaucoma.
• The participant has a positive test result for SARS-CoV-2 at the Baseline Visit.
• The participant has undergone a neurosurgical intervention for PD (such as pallidotomy, thalamotomy, fetal or stem cell transplantation or deep brain stimulation).

Note: Other inclusion and exclusion criteria may apply.