Two scientists have invented a kindness serum which can turn the fiercest human being into the sweetest kindest person. Testing the product on a wild lion proves successful. The "vaccination " becomes compulsory in the whole France, then in Europa. Mrs Durand ,who was a sour-tempered shrew, becomes tender and pampers her husband, happy as he has never been. The bossy colonel of the fourth floor surrounds his neighbors with attentions .But new problems appear, caused by this universal goodness:for instance, Mr Durand's boss's lover does not want to tell him she does not love him anymore for fear he may be grieved.Besides, a country in which people were not inoculated against nastiness becomes aggressive.