• Idiopathic PD with bradykinesia plus one of the other cardinal signs of PD (resting tremor, rigidity) being present, without any other known or suspected cause of PD untreated or treated with MAO-B inhibitor

• Body weight range between: \>/=45 kg/ 99 pounds (lbs) and less than or equal to (\</=) 110 kg/242 lbs

• Body mass index (BMI) of 18 to 34 kilograms per meter-squared (kg/m\^2)

• A diagnosis of PD for 2 years or less at screening

• Hoehn and Yahr Stage I or II

• A screening brain DaT-SPECT consistent with PD (central reading)

• Clinical status does not require dopaminergic PD medication and is not expected to require dopaminergic treatment within 52 weeks from baseline

• If presently being treated for PD, a stable dose of MAO-B inhibitor (rasagiline or selegiline) for at least 90 days prior to baseline and not expected to change within 52 weeks

• For women of childbearing potential: use of highly effective contraceptive methods (that result in a failure rate of \<1 percent \[%\] per year) during the treatment period and for at least 30 days (or longer if required by local regulations) after the last dose of study drug

• For men with female partners of childbearing potential or pregnant female partners, must use a condom during the treatment period and for at least 30 days (or longer if required by local regulations) after the last dose of study drug to avoid exposing the embryo. Men must refrain from donating sperm during this same period. The female partners should use a contraception method with a failure rate of \<1% per year during the treatment period and for at least 30 days (or longer if required by local regulations) after the last dose of study drug. Use of contraceptive measures is not required for male participants enrolled in Part 3.