Eddie pada usia hanya sekitar 20-an sudah merasa kejayaan urusan perniagaannya di Internet. Dia seorang jejaka yang memiliki segalanya – tampan, menikmati kehidupan mewah dan juga teman wanita sekeliling pinggang. Tanpa diketahui sesiapa, Eddie sebenarnya meraih keuntungan daripada perniagaan yang membeli dan menjual klip-klip pornografi dan sensasi di Internet. Maya Akhtar pula adalah seorang jurnalis yang komited terhadap kerjayanya. Malangnya, dia telah jatuh mangsa kepada ugutan Sunny yang memiliki rakaman video hubungan sulit mereka. Lebih mengejutkan lagi, perbuatan Sunny ini disokong oleh isterinya sendiri Rita, kerana inilah cara mereka membuat duit, dengan memperdaya wanita untuk tidur dengan Sunny dan memeras ugut mangsa kemudian. Suria, Mina dan Zakiah adalah tiga gadis remaja yang dalam perjalanan untuk mencari hiburan hujung minggu di Kuala Lumpur. Tanpa disangka, malam itu berubah menjadi suatu peristiwa yang menyeramkan apabila mendapati diri mereka diburu oleh satu makhluk yang mengerikan. Sandra adalah seorang gadis sekolah yang menjadi mangsa deraan seksual oleh Jefri. Lebih dari itu, Sandra satu hari telahdiperkosa beramai-ramai oleh satu geng motosikal yang dianggotai Jefri dan abang kandung Sandra sendiri iaitu Burn. Acara perkosaan ini telah dirakamkan oleh geng ini melalui kamera telefon bimbit Jefri. Dalam filem 3GP, kehidupan watak-watak berbeza ini akhirnya bersulaman dan berkait melalui pembocoran klip-klip 3gp mereka kepada masyarakat Internet –dan dunia mereka bertembung dalam satu kemuncak yang dramatik.

Seven Tulane University undergraduates – Sara, Nick, Beth, Malik, Maya, Blake and Gordon – drive to Sara's family vacation home on a private lake. There, Sara encounters her old boyfriend, Dennis and his friend, Red. Nick, Blake, Malik and Maya go wake-boarding when a shark attacks Malik. He is able to swim back to shore – missing his right arm. When Nick swims into the lake to retrieve Malik's arm, he is pursued by the shark, barely making it to shore. Being a pre-med student, he stabilizes Malik, and they all take him to the hospital. Blood from Malik's wound attracts a shark that attacks the boat. Maya (Malik's girlfriend who he was about to propose to) is knocked into the water and eaten. The shark damages the steering column of the boat, causing it to crash in front of the boathouse. Sara, Nick and Malik make it to shore. Dennis and Red arrive and agree to take Beth and Gordon to the mainland. During the boat trip, Dennis reveals that he and Red put the sharks in the lake and feed college students to them. Dennis forces Gordon into the water by shooting him; he swims to a mangrove tree but is devoured by a bull shark. Red and Dennis force Beth to strip down and feed her to cookiecutter sharks. Malik arms himself with a harpoon and ventures into the water to kill the shark that killed Maya. His wound attracts a hammerhead shark; he kills it but is further wounded. Nick and Blake pull the shark to shore, and Nick finds a camera attached to its belly. Blake decides to take Malik to the mainland on a jet ski. When they are pursued by sharks, Malik sacrifices himself and later Blake is killed by a Great white shark. Sheriff Sabin visits Sara and Nick and gives them soup, which makes Nick and Sara's dog, who has also eaten the soup, pass out. Sara hears Dennis and Red speaking from Sabin's radio, describing Beth's death. Dennis and Red then appear and take Sara to their boat, while Sabin prepares to dunk Nick into the lake, where sand tiger sharks wait. Sabin reveals that he, Dennis and Red have been inspired by the Discovery Channel's Shark Week and Faces of Death videos to film people being eaten by sharks. Nick is able to break free and ignite a can of gas. Badly burned, Sabin plunges into the lake and is eaten by the tiger sharks. Dennis reveals he is bitter at Sara for running him over with a jet ski, damaging half his face a while back. Red throws her dog into the water and soon after Dennis calls up Carl, the guy from the bait shop, telling him to release another great white shark, Dennis and Red lower her into the lake in a shark cage. As Dennis is about to release her, Nick arrives and takes Red hostage. After a scuffle during which Dennis kills Red in order to catch Nick offguard, Nick shoves Dennis into the water and tries to set Sara free but Dennis appears and tries strangling him. Nick escapes as a great white shark devours Dennis. Nick kills the shark and frees Sara. Nick, Sara, and Sara's dog swim to the boat, having survived. A distance away, a great white shark breaches, signifying that sharks remain in the lake