The episode in which Data's creator, Dr. Noonien Soong, appears is "Brothers." Data is unexpectedly summoned to Soong's remote laboratory on Terlina III, where he reunites with the scientist who built him.
Soong is dying and has called Data because he wants to give him an emotion chip. He accidentally summons Lore as well, believing that Lore is still disassembled. The reunion therefore becomes a tense family encounter involving Data, Lore and their creator.
Lore tricks Data, takes his place during the procedure and receives the emotion chip. He then injures Soong and escapes, leaving Data to confront the complicated meaning of brotherhood and his inability to experience grief in a human way.
"Brothers" is also notable because Brent Spiner plays Data, Lore and Dr. Soong in the same episode. It aired during The Next Generation's fourth season in 1990. "Datalore" introduces Lore and explains Data's origins, but it is not the episode featuring Soong in person.