From Longman Dictionary of Contemporary English(just) plain Mr/Mrs etc(just) plain Mr/Mrs etcspoken used to show that someone does not have or use a special title I don’t call him Uncle – just plain Bill. → plain
Examples from the Corpus
(just) plain Mr/Mrs etc• And being Lord Aviemore is just like being plain Mr Aviemore, right?• Cluedo's Reverend Green is to become plain Mr Green to bring one of the country's best-loved board games up to date. 