American English has always preferred the punctuated spellings, Mr., Mrs., and Ms., treating the first two as abbreviations of Mister and, though rarely spelled out, Missus. For the sake of consistency, Ms., though not an abbreviation, is also punctuated. British English does not treat these titles as abbreviations and leaves Mr, Mrs, and Ms unpunctuated. Miss, a contraction of mistress, is never punctuated.
Historically, Mrs. countered Mr. and was read as Mistress, the feminine of Mister, itself derived from master. Mrs. was originally used for both married and unmarried women but by the 1900s had become a title for married women only with Miss designating unmarried women.
The plural forms of Mr. and Mrs. take from the French Messieurs (Messrs.) and Mesdames (Mmes.). As with the singular titles, neither is punctated in British English.