 Route 28 crosses into Plainfield, Union County, where it becomes county-maintained Front Street, resuming northeast through residential and industrial areas before entering a more residential environment. The road encounters a mix of residences and businesses before intersecting Plainfield Avenue, where County Route 620 continues northeast on West Front Street and Route 28 turns to the southeast onto Plainfield Avenue. The route crosses the Raritan Valley Line before turning northeast on a one-way pair, with eastbound Route 28 following Fifth Street and westbound Route 28 following Fourth Street. County Route 603 continues southeast on Plainfield Avenue from this point. The one-way pair, which has two lanes in each direction, passes residences before crossing County Route 531 (Park Avenue) in the commercial downtown area of Plainfield, coinciding with the directional change of Fourth Street and Fifth Street from west to east. At the intersection with Roosevelt Avenue, Route 28 becomes 25 mph (40 km / h) two-way East Fifth Street, with the name becoming South Avenue at the intersection with Richmond Street. The route passes through urban residential and industrial areas, with the Raritan Valley Line located a short distance to the northwest. The road extends through more suburban commercial areas before crossing County Route 611 (Terrill Road), where Route 28 enters Fanwood as a 35 mph (56 km / h) road. A short distance later, at the second intersection with Old South Avenue, the route becomes state-maintained again and continues past businesses, intersecting County Route 655 (Martine Avenue) before it heads into residential neighborhoods and meets County Route 601 (Laurel Place).
