PRASSTHANAM is the story of family, trust and betrayal in a lawless land. 25 years ago, in Ballipar, Shiv (Anuup Sonii) has stood for elections against a violent rival. The rival stabs Shiv during a rally, resulting in his death. Shiv's father is an ex politico who is distraught by the demise of his son. He tells his most trusted aide, Baldev Pratap Singh (Sanjay Dutt) to take over the reins and also to take care of Shiv's wife Saroj (Manisha Koirala) and his two children. Baldev agrees but not before eliminating the rival candidate who killed Shiv. Baldev is helped here by his closest confidante, Baadshah (Jackie Shroff). Baldev marries Saroj and this angers Shiv's daughter Palak. But Shiv's son Ayush looks up to Baldev. Baldev and Saroj give birth to a son, Vivan due to which Palak further feels alienated.
The story then moves 25 years forward in the present day. Baldev has been a four time MLA and is busy with the upcoming elections. He recently got a stay order on a land which is being illegally usurped by a cunning businessman, Bajwa Khatri (Chunky Panday). Palak (Chahatt Khanna) is now a doctor and is happily married and also has two kids. She has completely broken contact with Saroj and Baldev and she only is in touch with Ayush (Ali Fazal). Ayush still has the respect for his step father and dutifully works under him. Baldev is so happy with Ayush that he's made it clear that he will be his heir. Vivan (Satyajeet Dubey) is infuriated with this bond as he feels he's the biological son of Baldev and hence he is the rightful heir.
Baldev however realises that Vivan is not cut for politics and advises him to study abroad and then handle his hospitality business. Vivan revolts and due to his hot headed nature, ends up insulting Asma (Divinaa Thackur), Baadshah's daughter who's also the manager in Baldev's hotel. Baldev is aghast with Vivan's behaviour and tells Ayush to apologise to Asma which he does. He also tells Ayush to train under Asma and he agrees. While working together, both get close. One day, Baldev is shot by assailants belonging to Bajwa who wants to avenge the loss of his land. Baldev survives and gets second thoughts about fighting elections. But Ayush takes care of everything and ensures that Baldev signs the nomination papers. Baldev gets whole hearted support from locals and hence, the party assures that he'll become the state home minister after winning the elections and that Ayush will be youth president. Vivan revolts but Baldev shuts him up. Vivan ends up drinking and creating a scene in the hotel. Asma tries to control the situation but he ends up drugging and raping her. When he and his friends escape from the hotel with Asma, their car meets with an accident. They decide to finish Asma and try to pass it off as a road accident. What happens next forms the rest of the film.