Tom and Barbara decide to sweep their chimney and,in the process,get covered in soot. They go next door to the Leadbetters' for a bath but subsequently discover that they have fleas, causing Barbara to act very strangely towards the postman. Fearing that they have brought an infestation upon Margo and Jerry, Tom calls in pest control expert Mr Bulstrode and he tells them that the fleas come from a dog - not just any dog, but the pet of a woman with whom Margo is canvassing for the Conservatives at the upcoming election.