Watch Undertow Full Movie Free Online 123Movies. Johnny Mack Brown stars as Paul, who wants nothing more out of life than to take charge of a lighthouse. Falling in love with Sally (Mary Nolan), Paul talks her into sharing his life as a lighthouse keeper. Evidently staring into the beacon once too often, Paul goes blind, and it's quite a chore for footloose Sally to remain faithful. Making matters worse is the arrival of a double-dyed villain (Robert Ellis) who intends to "have his way" with the long-suffering heroine.