The Partridge family discovers that during the night, someone has entered their house and burglarized their TV, radio, and stereo. As a gift, Reuben has a burglar alarm installed in the Partridge home. It has sensor pads in various places under the carpeting -- one step on a sensor and the house is filled with a heart-stopping, loud, clanging bell. While the Partridges are trying to adjust to the alarm and remember to turn it off when they are in the house, the neighbors are getting more furious each time it is set off accidentally. Petitions are being signed asking that the alarm be removed. When Shirley goes to the garage to shut off another false alarm, she finds Morris Dinkler, an escaped con who holds the Partridge family and Reuben hostage. He purposely chose their house because of the alarm, knowing that new alarms are frequently set off by accident, and after a while, the police don't bother to come. Everyone assumes the bulge in Morris's pocket, which he refers to as Big Bertha, is a gun; it turns out to be a harmonica! Morris is really quite a nice guy who had a bad break. He only had four months until his parole when he was taken along as a hostage by escaping prisoners. He is convinced the police won't believe him. In order to set a good example for the younger kids, Morris turns himself in to the police with the promise of help from the Partridges. When he surrenders, Morris learns that the police know he was innocent as far as the breakout was concerned.