The Mysterious Mr. M is a 1946 Universal Pictures movie serial, the 137th and last serial produced by Universal.

Plot
Anthony Waldron intends to steal a new submarine invention from Dr. Kittridge while blaming a fictitious mastermind he calls “Mr. M.” To further this plan, Waldron uses a mind control drug he has developed called “Hypnotreme.” However, a mystery villain soon appears claiming to be the real Mr. M and starts giving Waldron orders.

Federal agent Grant Farrell, whose brother was killed by Waldron, is dispatched to find the mysterious villain and stop his nefarious plans, teaming up with Kirby Walsh and Shirley Clinton to do so.

Cast
Richard Martin as Detective Lieutenant Kirby Walsh
Pamela Blake as Shirley Clinton, insurance investigator
Dennis Moore as Agent Grant Farrell
Virginia Brissac as Cornelia Waldron
Danny Morton (actor) as Derek Lamont, one of Waldron’s henchmen
Edmund MacDonald as Anthony Waldron, the original villain
Byron Foulger as Wetherby
Jane Randolph as Marina Lamont, one of Waldron’s henchmen
Jack Ingram as William Shrag, the spearpoint heavy (chief henchman)