The premise, as others point out, is wildly improbable, yet the visuals, direction, and acting prove enough to compensate. Poor Chick Graham (Nelson, in a dual role). On an utterly routine work day he returns home, only to find an exact double has taken his place in the household. Even his wife (Ainley) says the real Chick is an impostor, and the officials believe her, not Chick. In fact everything he does to verify his identity backfires, including fingerprints. So what's going on since Chick's just an ordinary working stiff.
Frankly, I wish the screenplay hadn't tipped its hand so soon by explaining the reason behind the impersonation. Withholding the key would have set up a good mystery. Nonetheless, Nelson's dual role is well handled. Catch the vertical line created by door-frames that indicates where the split screen divides when the two Nelsons appear in the same shot. The split screens are well done without being obtrusive. Also, the climactic chase through the old Spanish coastal fort is excitingly picturesque. In fact, the movie makes good use of the Puerto Rico locations, even if in b&w. These lend an exotic flavor to an exotic story.
All in all, the movie certainly doesn't lack for imagination, including the assassin canine and its scary handler (Jim Boles). Speaking of the Doberman, I'm glad the besieged Chick finally figures out the tactical use of a door. Anyway, with some basic work on the screenplay, this could have been a B-film that's more than novel entertainment, which it is.
