Although most of us worry from time to time, there are people whose vivid imagination causes them to live in an almost permanent state of apprehension and alarm. For them the chink of a milk bottle in the small hours is never a prowling cat, must be an incredibly clumsy burglar. The footsteps that follow them down their quite road are never those of a patrolling policeman, no, no, no they must belong to Jack the Ripper or Sam the Strangler, probably both. The natural leader of this unhappy band of apprehensive brothers is Chief Petty Officer Pertwee. Anything he doesn't already know about or understand is a dead cert to be stuffed full of utter doom and disaster.