I saw the series on DVD; there are 4 series with the terrific Michele Placido in the lead as solitary and stubborn police inspector Corrado Cattani, who is determined to conquer the evil elements in Italy.
To start with, it's interesting to see the dramatic quality increase of the production. In the first series the sound and image quality is terrible, yet in the fourth everything seems to be at a higher level. More attention was paid to the cinematography, it looks more professional and there are more dramatic "tricks" than in the previous series. Even the actresses look more beautiful! Patricia Millardet plays the mature and strong judge Silvia Conti to work aside Cattani while Simona Cavallari plays the juvenile Esther Rasi who idolizes Cattani) Next to being a thriller, La Piovra is also convincing as a study of organised crime in Italy. The characters are well developed and the acting is fantastic. The basic plot line: inspector Cattani's eternal conflict of interests between his family life and his professional life, which sometimes leads to very dramatic scenes. He loves his family yet his urge to combat organised crime in his beloved country is equally strong. At various points he is "accused" of being a moralist. Actually he is one of the few that dares to makes a fist against the bunch of opportunists who seem to have infiltrated Italy's political and economic system at all possible levels... The tormented inspector fights different enemies; my favourite one is Tano Carridi (4th series, very well played by Remo Girone). He displays a great mix of villainy and sadness that almost makes you symphatise with him. A worthy opponent of Cattani! The great music by Ennio Morricone in the series also adds a lot to the atmosphere! I highly recommend this series.