Eight years following the dramatic incidents of the first film, Dr. K. Vaseegaran introduces an advanced humanoid android, Nila, to a group of eager college students. During this presentation, he reflects on his earlier creation, Chitti, which was regretfully decommissioned after going on a deadly rampage. However, chaos ensues when mobile phones across Chennai mysteriously lift from people's hands, causing widespread panic. In response to this alarming event, a high-level emergency council convenes, consisting of various political figures and scientists, to deliberate on the crisis at hand.
Vaseegaran advocates fervently for the reactivation of Chitti to address the growing threat, but he faces staunch opposition from council member Dhinendra 'Dhina' Bohra, who harbors a strong personal grudge against Chitti due to a past incident where his father, Professor Bohra, was killed. As the situation escalates, mobile phones begin to gather into a colossal swarm that violently eliminates notable telecom executives and corrupt officials, prompting the Home Minister to reluctantly grant permission for Chitti's reactivation.
Once reactivated, Chitti confronts the swarm, which has now taken the form of a gigantic bird, wreaking havoc upon the city. As he battles this formidable foe, Chitti's power begins to dwindle, forcing him to retreat to a nearby space research facility to recharge. The swarm attempts to follow but is thwarted by the high-frequency positive ions emanating from the station's antennas.
Upon investigating the phenomena, Vaseegaran and Chitti uncover that the mobile devices are intertwined in a mass of negatively charged energy with volatile electromagnetic properties. In a bid to exploit this weakness, Vaseegaran constructs a powerful photon synthesizer capable of emitting positive ions. With this new technology, Chitti manages to neutralize the swarm's negative charge in a critical showdown, causing it to collapse. However, the remaining phones transform into the form of Pakshi Rajan, a troubled ornithologist.
Through dialogue with Pakshi, Chitti learns of his harrowing history as a passionate conservationist who operated a bird sanctuary. Pakshi actively campaigned against the unchecked spread of cellular networks, which he believed were devastating local bird populations due to the harmful radiation from cell towers. His cries for help were ignored by powerful telecom corporations and corrupt officials, leading him to despair and ultimately take his own life from a cellular tower. Subsequently, his essence merged with the life force of the birds he loved, turning him into an avenging spirit capable of manipulating mobile phones.
Even after Chitti pleads for peace, Pakshi's intentions remain clear: he aims to annihilate humanity to exact revenge for the destruction of his bird sanctuary. In a pivotal moment, Chitti successfully captures the negative energy within a specialized containment circuit. Following this event, the government commissions Vaseegaran to develop an army of androids for protection. However, Dhinendra, still embittered, infiltrates the lab and releases Pakshi, who then seizes control of Vaseegaran's body and initiates another attack on the public.
In an intense confrontation, Chitti battles his possessed creator but chooses not to resort to deadly force, leading to his own dismantling by Pakshi. Meanwhile, Nila, following Vaseegaran's coded commands, manages to recover Chitti’s remnants and retrieve the controversial "red chip" tied to his former violent behavior. By reprogramming the chip to target only Pakshi, Nila upgrades Chitti to the combat-focused version "2.0".
Pakshi, now posing a more significant threat, traps a multitude of people within a sports stadium and uses the nearby cell towers to execute Dhinendra through concentrated radiation. Chitti and the newly formed robotic army arrive to combat Pakshi, but they face overwhelming odds as he constructs wings and bird drones from the phones. Just as Pakshi prepares to execute the hostages, Chitti unleashes Kutti "3.0," a fleet of miniature cellular androids that attach to the phone structures and self-destruct, forcing Pakshi to retreat and allowing the hostages to escape.
Ultimately, Chitti and the remaining robotic units manage to lure Pakshi back to the research station, where he is finally defeated using the positive ions from the antennas. After Vaseegaran recovers in the hospital, he passionately urges for stricter environmental regulations on cellular radiation. Now recognized as protectors of public safety, Chitti, Nila, and their robotic companions embark on government service. In a whimsical post-credits scene, Vaseegaran's personal smartphone unexpectedly transforms into a sentient Kutti microbot, humorously declaring itself the scientist's grandson.
