There are two stories about how Plain got its name, you may pick the one you like best.
It is a small town (north of Leavenworth) is near Lake Wenatchee - which is no where near the town of Wenatchee, WA, which is famous for its apples. The state wanted to name the small town near the lake, Lake Wenatchee, after the lake, but the town's people said please don't do that, then people will all go to the town of Wenatchee and we'll be forgotten up here. Please give us a plain name - and someone, with a sense of humor gave them the name of Plain, WA.
The other story is, in the late 1800's to early 1900's, Plain was called Beaver Valley, and was settled by the Church of Brethren. The postal service at the time did not accept the name Beaver Valley, claiming it was too long of a name, so the residents asked the postmaster for a "plain" name, thus Plain was born.
The name Beaver Valley is still seen on some of the businesses, so that might be a more accurate story - but either one is entertaining - and this cute donkey was very entertaining the day we were in Plain, WA