The movie ‘Oru CBI Diary Kurippu’ opens with DySP Prabhakara Varma observing the local police force during a training drill. Another officer approaches him and informs him that he is needed at a recent crime scene, and they depart immediately. Varma arrives at the home of Ouseppachan, a local businessman, and sees the body of Ouseppachan’s daughter-in-law, Omana on the grounds behind the house, in an apparent suicide. He is then introduced to Omana’s husband Sunny, her father-in-law Ouseppachan, Ouseppachan’s son-in-law Johny, Ouseppachan’s best friend Narayan, and several of the household servants and neighbors. The initial report conducted by the first responders concluded that Omana had committed suicide by flinging herself from the roof of the two-story house onto the stone pavement below. Varma questions the members of the household as well as neighbors and servants about Omana’s relationship with her husband Sunny. Sunny admits to arguing with Omana the night she died and throwing her onto their bed before leaving to drink, but swears he never hit her. He also tells Varma that Omana had attempted suicide before. Ouseppachan claims he came home from a business trip that night, dropped off by his driver, Vasu, only to find himself locked out of the house. He decided to go around the back to where the cook’s quarters were located when he stumbled upon Omana’s body. Dissatisfied with inconsistent testimonies from several suspects, Varma immediately suspects foul play and is determined to find the truth. The next morning, the Regional Medical Officer concludes that Omana died from head trauma at 8pm the previous night.
Omana’s cousin Chacko, a police officer working under Varma, finds this autopsy report suspicious and tells Varma and Officer Alex why he has doubts to its accuracy. He informs them that it rained severely until 9:30pm the night Omana allegedly committed suicide, and if she had died at 8pm, her corpse would have been wet, and the blood from her injury would have been mixed in with the rain water. Alex, who is taking bribes from Ouseppachan, warns Ouseppachan to do what he can to get Varma and Chacko off the case.
Sensing that Varma will cause him trouble, Ouseppachan implores his best friend, Narayanan, a local politician, to call his politician friends to have Varma transferred off the case and onto Rural. They also managed to transfer Omana’s cousin, Chacko to Traffic. A new Dy SP, Devadas, takes over the case. Devadas, who is also being bribed by Ouseppachan, intimidates Omana’s grieving family and any witnesses that may implicate Ouseppachan or Sunny. He quickly concludes the investigation stating that Omana committed suicide.
Omana’s father Thomachan and her sister Annie are determined to seek justice for Omana. They implore the CBI to re-open the case and find the truth. The Director of the CBI assigns Sethuraman Iyer to head the investigation. He is assisted by Sub-Inspector Harry and Sub-Inspector Vikram.
Iyer and his team find evidence of political interference in Omana’s murder investigation, citing an increased amount of phone calls to Ministers and Party Leaders from Narayan’s phone number, and thousands of rupees worth of cash withdrawals from Ouseppachan’s bank that were never reported in the books. Iyer meets with Varma and Chacko who help him with his investigation by informing him of inconsistent testimony from suspects and the inconsistencies with the Regional Medical Examiner’s autopsy report. Later that week, Iyer and his team bring two dummies the same height and weight as Omana to Ouseppachan’s house. They take the dummies to roof and drop one straight down as if it jumped, and threw the other off the roof as if someone had thrown the body. They find that Omana’s body landed approximately around the same place as the dummy that was thrown, and concluded that she could not have jumped that far off the roof on her own. Next, they decide to track down Ouseppachan’s driver, Vasu, who has been missing since the morning after Omana’s death. Harry finds out that Vasu had given a local banker some jewelry as collateral for a loan. Among the jewelry was a bracelet belonging to Omana, which immediately put all suspicion on Vasu. They finally find him and take him into custody, where he confesses that shortly after he got home after dropping off Ouseppachan the night of Omana’s death, Narayan came to his house and told him that Sunny had accidentally killed Omana during a domestic dispute. Vasu then said Narayan and the others implored him to carry Omana’s body from her bed to the roof and fling her off. It was during a moment alone with Omana’s dead body that Vasu had noticed her gold bracelet and decided to steal it, before throwing her corpse of the roof.
Despite Vasu’s testimony implicating Sunny as the killer, Iyer is not convinced. Sunny had left the house at 8pm after hitting Omana and that would mean that she died at 8pm. However, Iyer examines photographs of the crime scene and finds that Omana still bled when she was dropped off the roof some 2 hours later, and since dead bodies do not retain heat thus causing the blood to clot, she could not have died at 8pm. Upon scrutinizing the photographs further, Iyer notices a small blood stain on Omana’s saree below the waist. Since the autopsy report stated that she sustained no injuries below the waist, Iyer is convinced that the blood was left behind by the real killer.
After testing the blood they find that it is O-negative, a rare blood type. Remembering that the town had imposed mandatory blood tests on all its citizens recently by the local authorities to combat malaria, Iyer goes to the Malaria Research Center and finds that the blood type was so rare that only two people in the entire town had it.
In the final scene, Iyer arrives at Ouseppachan’s house with a large police backup unit and places Ouseppachan under arrest for the murder of Omana. Shocked, Ouseppachan vehemently denies killing her, but Iyer would not relent. Seeing his father being arrested for a crime he committed, a guilt-ridden Sunny confesses to Iyer that he killed Omana accidentally by hitting her too hard when they fought. Iyer tells Sunny that Omana did not die when he hit her, that she only fainted. Iyer then goes on to explain the blood on Omana’s saree, which did not match her own, was left behind by the real killer. He informs them that only two people in the entire town were O-negative and one of those two people is Johny, Ouseppachan’s son-law. Confronted with evidence for his involvement, Johny confesses that he had come to the house the night Omana had died. Finding her passed out on the bed, he attempted to rape her, but when she woke up and started screaming, he suffocated her to death. In the process he accidentally cut himself and left his blood on her saree. Thomachan and Annie thank Iyer, Harry and Vikram for their hard work and dedication and bringing Omana’s killer to justice. The movie ends with a reference to Sukumaran Kurup, a real life notorious fugitive wanted in India for murder, who to this day is still on the run.