Most of the documents I edit in Google Drive are plain text. However, since the Google Docs app is basically MS Word, it keeps trying to format all the text formatting I try to add. There is no plain text mode for Google Docs. It is for editing and maintaining plain text files that the webapp Drive Notepad is perfect for. It is essentially MS Notepad for Google Drive.
You can visit the Drive Notepad website and click on Launch App to start it. Google will get your permissions the first time you use this app.
Drive Notepad uses a hovering taskbar that shows the current status of your document. Click it and it expands to shows all the buttons in the taskbar. You can basically Save, New or open files from Google Drive. Note that you need to explicitly save the document by pressing
Ctrl + S. There is no auto-save.
You can also open any existing
.txt file from Google Drive using Drive Notepad. Google Drive just displays the text in a read-only view, but look for the dropdown at the top to open in Drive Notepad for editing.