The Sethna family of Panchgani consists of retired sculptor Dinshaw (Naseeruddin Shah) and his wife Katy (Dimple Kapadia). Dinshaw's brother Farrokh (Boman Irani), their father Fardoonjee (Honey Chhaya), and Farrokh's wife Tina (Simone Singh) reside in Mumbai.

It is at this time that Cyrus Mistry (Saif Ali Khan), the narrator of the story, shows up at the Sethnas' doorstep in Panchgani one morning, a seeming wanderer hailing from Jamshedpur who professes his admiration for Dinshaw's work and asks if he can be his apprentice. Dinshaw and Katy agree, and soon the young man becomes a part of their life. Katy makes advances towards Cyrus, which he tolerates while cryptically telling the audience that he is "playing Katy". As time passes Cyrus gets to know the oddball couple better – Dinshaw is a perpetually zoned-out eccentric, though a rather affable one, while the rather promiscuous Katy, tired of slow-moving Panchgani life and her moth-eaten marriage, has embarked on an affair with her brother-in-law Farrokh in Mumbai. Katy makes Cyrus her pawn, sending him to Mumbai to visit her father-in-law Fardoonjee, who lives in an extension of Farrokh's flat, with orders to get a stash of money Fardoonjee supposedly has. On meeting Fardoonjee, Cyrus realizes that he is ill-treated by Farrokh. Farrokh's timid young wife, Tina, is unable to be herself except during the few hours Farrokh is at work. She enjoys smoking, loud music and goes to hair appointments when Farrokh is gone.

One day Katy convinces Cyrus to go and murder her father in law and Tina so that she can be with Farrokh. Unknown to Cyrus, Farrokh plans to kill him after the deed to close the loose end. Cyrus arrives at a time when Tina is coincidentally out for a waxing appointment. He feeds Fardoonjee his favorite dish and kills him immediately afterwards. This is positioned to the audience as a mercy killing to release Fardoonjee from his wretched existence. Farrokh sneaks into the house but Cyrus kills him instead. When Katy calls to check on Farrokh, Cyrus picks up the phone and tells her that her plan is in tatters. And then Cyrus vanishes.

The Inspector calls Katy and Dinshaw for interrogation. She constantly blames Cyrus for the murder. When the Inspector investigates, he finds no evidence of any person named Cyrus even in the market in Panchgani. So Katy and Dinshaw are arrested for the murders while poor Tina is sent home with both her and the Inspector agreeing that she was lucky to be away at that time.

A few months later, Tina is in her bedroom when the door is opened surreptitiously by Cyrus. It looks like he has come to finish the last murder. Shockingly, Tina and Cyrus embrace each other. They are siblings. This whole murder is shown to be an elaborate plot by Tina to inherit the whole property. With Fardoonjee and Farrokh dead, and Katy and Dinshaw in Jail, her plot has been successful. Tina is shown as a cold-blooded schemer who identifies such families, gets married into the family and has Cyrus murder the members. Cyrus is shown to have a conscience. While Tina is showering, he sees a newspaper where Tina has marked another family for this scheme. Unwilling to participate in this any further, he runs away with the money they took before Tina comes out of the shower.

• Saif Ali Khan: Cyrus Mistry/Xerxes
• Naseeruddin Shah: Dinshaw Sethna
• Dimple Kapadia: Katy Sethna
• Boman Irani: Farrokh Sethna
• Simone Singh: Tina Sethna
• Honey Chhaya: Fardoonjee Sethna
• Manoj Pahwa: Inspector Lovely

At the 52nd Filmfare Awards held in 2007, Being Cyrus received 7 nominations, all in the technical categories. This was the most number of nominations for a film that did not win any awards. At the 2nd Global Indian Film Awards, the film received 3 nominations; 2 for Adajania and 1 for Kapadia.

Being Cyrus received mixed reviews from critics. Critic Rajeev Masand rated the film 3 out of 5, praising the performances and story. The Times of India rated the film 3.5 out of 5.^[6] Anupama Chopra from India Today termed the film as "Being Cyrus is a competent, well-crafted debut."^[7] Indrani Roy Mitra from Rediff.com rated the film 3 out of 5, terming it as "a unique attempt".^[8]

Poonam Joshi from BBC rated the film 3 out of 5, terming it as "The emphasis here is on strong storytelling and riveting visuals, making for a curiously original film."^[9]