The Theatre Building, which included 2 levels of shops and office spaces, was officially dedicated in late 1928. 
Photo from State Historical Society of Missouri / J.C. Nichols Company Scrapbook

An interior view of the posh Plaza Theatre. It was modeled after the Palacio de las Duenas in Seville, Spain.
Photo from State Historical Society of Missouri / J.C. Nichols Company Scrapbook

The Plaza Theatre showed its first feature in October 1928. On the opening night bill was Janet Gaynor starring in "Street Angel." The Fox Studio release (filmed before the mid-1930s merger of the 20th Century and Fox Studios) featured a Movietone soundtrack. This was a technological stepping stone between silent films and "talkies." There was no recorded dialogue, but interludes of music and sound effects.
Graphic from Wikipedia / "Devinn"