Based on the multi award-winning and critically acclaimed movie of the same name, Mafia Only Kills in Summer mixes tragedy and comedy to tell the story of an Italian family, beginning in late 1970s Palermo, against a setting of growing mafia violence.
Salvatore is a 10-year-oldboy. One day a new classmate, Alice, arrives and he falls head over heels in love with her. In Palermo, people say you can die for love of a woman, and Salvatore is soon convinced that if he asks Alice to go out with him he will die; so he lets his best friend Fofò ask her out instead. In the meantime, Salvatore's father Lorenzo witnesses a Mafia murder and doesn't know what to do about it.
After giving it a great deal of thought, Lorenzo decides the time has come to move house, so he starts looking for a new home. But moving house isn't a simple matter in Palermo, and he soon comes up against the construction industry Mafia. In the meantime, Fofò has become Alice’s boyfriend and hasn't died. So Salvatore decides to try to win over Alice, who he has been in love with ever since he first saw her.
Anti-mafia journalist Mario Francese goes to Salvatore's school to explain what his work involves. Thanks to him, Salvatore discovers the dark side of Palermo, and when the journalist suggests the children might like to try conducting an investigation of their own on any subject they like, he decides on the water shortages in Palermo. The day before the project is due to be handed in, Mario Francese is murdered. Meantime, Lorenzo's mortgage application is turned down and, for the time being, his dream of buying a house evaporates.
Uncle Massimo has an idea to turn things around: he wants to sell the land at Rocca Busambra, which belongs to his father, Tore. The whole family travel to Ficuzza to try and convince grandpa to sell, but he refuses to budge. The fact is that this land hides a terrible secret: one which concerns his own past, and that of all Sicily.
Salvatore keeps a secret diary where he records all his worries and doubts, but he mostly ends up writing about Palermo and the Mafia. Lorenzo and Pia find out and are concerned, so they decide to take him to see a psychoanalyst. But Lorenzo, too, has doubts of his own: his office manager has died and he is next in line for his job, but he is afraid to accept it because he knows his boss used to give in to the Mafia’s demands, and he fears he will have to do likewise. In the meantime his older sister Angela breaks up with her boyfriend Rosario.
After her break-up with Rosario, Angela thinks she's finally found the love of her life. His name is Alfonso Puccio and he's the son of one of the biggest car dealers in Palermo. Her life seems finally to have taken a turn for the better, but Lorenzo finds out that Alfonso's father isn't just a car dealer; he's actually a front man for the Mafia. Meanwhile Salvatore decides the time has come to declare his love for Alice, and Pia finds out her job is likely to become permanent; her life is finally about to change for the better.
While Massimo and Patrizia are planning their wedding, Pia tries to find out why she wasn't actually offered a permanent position in the end. Thanks to Ayala, a work colleague who has been in love with her for a long time, she discovers that the selection process is actually rigged. Overcoming his reluctance, Lorenzo asks Massimo for help in seeking justice for his wife.
Massimo has ended up in prison because the person he asked for a 'favor' to help his sister Pia was actually being investigated. In prison, he strikes up a friendship with one of the most feared and respected bosses in Palermo, Tommaso Buscetta, who turns out to be an old acquaintance of Pia. Thanks to 'Don Masino', Massimo obtains permission to marry Patrizia in prison, but Lorenzo doesn't want to know about taking part in the ceremony. He only agrees for love of Pia.
All Palermo is anxiously awaiting the match which could change the city's fortunes: Palermo-Juventus, the final of the Coppa Italia. Lorenzo, too, is confident about his family's future on this evening, mainly because thanks to Alice's father, who is an important banker, his mortgage has been fast-tracked. But Palermo loses the match, and all hope for a better future for the citizens of Palermo, including Lorenzo, evaporates with the team's defeat. In the meantime Massimo is starting to realize the price he will have to pay for joining Cosa Nostra, the Mafia.
The political elections make an impact on the daily lives of the citizens of Palermo. While his relationship with Pia becomes more tense due to jealousy and his suspicion that she is being unfaithful, for the first time Lorenzo accepts the idea of putting himself in a compromising position in order to achieve his dream of a new home. Massimo, on the other hand, is ordered to carry out a crime and finally realize she's not cut out for the Mafia.
The Giammarresi family finally take up residence in their eagerly awaited new home, but Lorenzo has to bear the heavy weight of his conscience because the house is a constant reminder of the compromise he had to reach. Salvatore has to deal with Sebastiano's unhappiness at discovering his father's secret, thanks to Fofò, and must also try to prevent Alice leaving Palermo for Switzerland. He must tell her he loves her before it's too late. He asks his mentor Boris Giuliano, for whom the Mafia has put out a death warrant, for help. The morning of the murder, Salvatore sees something he should never have seen.
Palermo and the Giammarresi family are coming to terms with the tragic death of Boris Giuliano. Lorenzo, on the other hand, must deal with Salvatore's terrible secret; he saw something linked to the murder and wants to testify on behalf of his friend Boris. Patrizia is convinced Massimo is being unfaithful to her, but in fact her man is concealing a far more painful secret. Alice is almost ready to leave Palermo. To persuade her to stay, Salvatore decides to put the advice Boris Giuliano gave him before he died into practice.