24

In 1990, Sethuraman is a scientist and renowned watchmaker who lives with his wife Priya and newborn son Manikandan. On 13 January, he completes his invention of a time-travelling watch: Project 24. His happiness is short-lived, however, as his gangster twin brother Athreya attacks the family and kills Priya to get the watch. He only finds the key to the box where the watch was hidden. Unable to use the watch and fearful for his son's life, an injured Sethuraman manages to escape with his son and gets on a train, with Athreya in close pursuit. Sethuraman pleads with Sathyabhama, a woman he meets on the train, to take care of his son, and goes to face Athreya. He manages to trick Athreya into thinking he is carrying a bomb, prompting Athreya to quickly kill Sethuraman before jumping off the train to his apparent death. It is shown that Sethuraman placed the locked box containing his watch, along with Mani, who grew up unaware of its contents, and his true parents.

26 years later, in 2016, Manikandan is now a talented watch mechanic who believes Sathyabhama to be his mother. Mani meets a girl Sathya, who had come for her watch repair, and falls in love with her. Meanwhile, Athreya, who survived his fall and was in a coma, wakes up after 26 years (due to a fire in the hospital, in which the key to the box is dropped) and is shocked to find out that he is now paralysed from the waist down and older. He is cared for by his trusted confidante, Mithran.

In a chain of events, the key to the box ends up in Mani's hands. He opens the box and discovers the watch inside. He figures out its capabilities, including its ability to freeze time for 30 seconds, but questions the watch's origins and how it ended up in his possession.

Athreya, upset with the fact that he lost 26 years of his life and has become a paraplegic, obsesses over the watch and wishes to go back in time to relive his lost youth. Mithran issues an advertisement offering INR 5 crores (50 million) as a reward for the lost watch. This advertisement is discovered on a newspaper by Mani's friend Saravanan. Mani deduces that the ad creator is aware of the watch's powers. To find out the truth, Mani uses the original to make an identical copy and, along with Saravanan, goes to the provided address.

At the office, Athreya spots Mani's copy (his "watch" was the only one with the original back) and realises that Mani has the original watch. He summons both of them to the office and viciously attacks them, brutally killing Mani by chopping off his hand, and gets hold of the watch. An ecstatic Athreya reverses time, only to find out that the watch cannot transport its wearer beyond 24 hours, waking up at 12:00 A.M. the night before. He now realises that Mani is the only one who can upgrade the watch. Mani is then shown to be alive. Athreya decides to convince Mani that he is Sethuraman and pretends that he is dying from an incurable disease.

Now disguised as Sethuraman, Athreya finds Mani and tricks him into upgrading the watch to enable larger time jumps. Mani manages to upgrade the watch successfully but learns Athreya's true identity and the reason for his parents' deaths. He tricks Athreya into revealing the date of his parents' death and returns to 1990 to warn them. Unfortunately, right before he travels, Athreya manages to grab onto the watch, and they are both transported back: Athreya as a healthy adult about to storm Sethuraman's house, and Mani as a toddler who cannot speak. However, Sethuraman discovers the upgraded watch in Mani's crib. At first, he is excited that Mani time-traveled, but Priya asks why Mani would travel back to a time when he could not talk. Sethuraman quickly realizes, to his and Priya's horror, that Mani traveled back in time to warn them. With this realization, Sethuraman uses his watch to freeze time, kill Athreya, and escape his henchmen. Priya asks Sethu to throw the watch away as she has had enough of the past, and he complies.

Sethu and Priya board the same train as Sathyabhama, and they start a conversation in which Sethu tells Sathyabhama that he is a science teacher. Sathyabhama asks them to join her family's school, as they want to hire a science teacher. Sethu and Priya, making a fresh and happy start, agree.

In a mid-credits scene, a young Mani talks to a young Sathya.