The origin of the village's name is widely rumored to have been selected as an homage to the Shrine of the Virgin Mary at Maria Plain, in Salzburg, Austria. There are, however, no known official documents nor any newspaper articles or books of that time to support this claim, and writings of the time indicate a less colorful origin: In the book "Baraboo and Other Place Names in Sauk County, Wisconsin" (written in 1912, the same year that Plain was incorporated) it says of Plain that it was "called Plain because the inhabitants were plain people." In a September 23, 1915 letter in the local newspaper, the Weekly Home News, a subscriber wrote of his desire to have Plain re-christened, as the town had expanded and improved so much over the past three years that it had outgrown the "plain"ness of its name. In the letter, the author writes: "Within a few weeks very strong efforts will be made at proper headquarters to have the name of Plain changed, as that name does not agree with the rushing strides our burg is making. First of all there is no meaning to the word Plain, as it is an adjective; we must have at least a noun and why not put a "ville" or "city" to it."
There were 333 households out of which 31.8% had children under the age of 18 living with them, 58.9% were married couples living together, 5.7% had a female householder with no husband present, and 32.7% were non-families. 28.5% of all households were made up of individuals and 14.1% had someone living alone who was 65 years of age or older. The average household size was 2.38 and the average family size was 2.96.
In the village the population was spread out with 26.3% under the age of 18, 5.3% from 18 to 24, 27.4% from 25 to 44, 21.2% from 45 to 64, and 19.8% who were 65 years of age or older. The median age was 39 years. For every 100 females there were 102.0 males. For every 100 females age 18 and over, there were 96.6 males.
The median income for a household in the village was $44,028, and the median income for a family was $51,094. Males had a median income of $34,236 versus $25,385 for females. The per capita income for the village was $24,658. About 0.9% of families and 2.6% of the population were below the poverty line, including 2.1% of those under age 18 and 4.0% of those age 65 or over.