In 2013, an interdimensional portal called “the Breach” opens at the bottom of the Pacific Ocean, from which giant monsters, the Kaiju, emerge, destroying various cities across the Pacific Rim. In response, humanity builds massive robots called Jaegers to combat the monsters. Each Jaeger is piloted by two to three pilots, who are mentally linked in a process called “drifting” to share the mental stress of piloting the machine.
In 2020, brothers Yancy and Raleigh Becket pilot the Jaeger Gipsy Danger to defend Anchorage from a Category-3 Kaiju codenamed “Knifehead”. During the confrontation, the Kaiju unexpectedly pierces the Jaeger’s armor, ripping out its left arm and half of its head, which pulls Yancy out of the Conn Pod, killing him. Raleigh, piloting the damaged Jaeger solo, kills Knifehead and walks Gipsy back to shore by himself before collapsing. Traumatized by the loss of his brother and the strain of drifting alone, Raleigh quits the Jaeger program.
Five years later, in response to the increasing power and number of Kaiju attacks and despite the protest of the Jaeger program director Marshal Stacker Pentecost, world leaders decide to cease funding for the Jaeger program and replace it with coastal defense walls, believing it to be a more effective option. However, during a subsequent attack on Sydney, a Kaiju demolishes the city’s coastal defense wall and proceeds to wreak havoc and destruction, before being stopped by the Jaeger Striker Eureka. Defying their decision, Marshal Pentecost moves the remaining four Jaegers to the Shatterdome: the last remaining Jaeger base in Hong Kong to continue fighting the Kaiju. Determined to bring about the destruction of the Kaiju once and for all, Pentecost plans to destroy the Breach with a nuclear bomb.
Meanwhile, Raleigh is engaged in the coastal wall-building program. Pentecost tracks him down and persuades Raleigh to come back to the Jaeger program. At the Hong Kong Shatterdome, Raleigh meets Mako Mori, Pentecost’s adopted daughter and director of the Jaeger restoration program. Raleigh and Mako are found to be compatible pilots, but Pentecost is against Mako entering the Jaeger program, however Mako convinces him to permit her to copilot the restored Gipsy Danger. During their first drift test, Mako gets lost deep in the memory of her parents dying in a Kaiju attack, known as “chasing the rabbit.” Believing that she is going to destroy the Kaiju, Mako activates Gipsy’s plasma cannon, which threatens to destroy the Shatterdome, but Gipsy gets shut down before that can happen. Because of this, both are subsequently relieved of combat duty. While in the drift, Raleigh saw within Mako’s memories that it was Pentecost who saved her from the Kaiju attack that killed her parents, which explains why he won’t let Mako pilot a Jaeger. Armed with this knowledge, Raleigh confronts Pentecost and tries to persuade him to bring Mako back onboard.
Pentecost consults the Kaiju experts, Newton Geiszler and Hermann Gottlieb, to plan the assault on the Breach. Newton was able to drift with a piece of a Kaiju brain, but only saw fragmented memories. It is revealed that Kaiju are in fact genetically-engineered by a race of extraterrestrial beings who are using them to conquer Earth. He proposes finding a “fresh” brain, and under Pentecost’s advice, Newton meets Hannibal Chau, a black market dealer of Kaiju organs. When Newton tells him that he drifted with a Kaiju, Chau explains that due to the two-way nature of drifting, the brain Newton had drifted with accessed his memory as well. Since all the Kaiju share collective memories, they now have some of his knowledge.
Two Category-4 Kaiju emerge, Otachi and Leatherback, approach Hong Kong to find Newton. The humans deploy three Jaegers, Crimson Typhoon, Cherno Alpha, and Striker Eureka to intercept them. The creatures destroy Cherno Alpha, Crimson Typhoon while also disabling Striker Eureka with an EMP blast. Otachi proceeds to rampage through the city in an attempt to find Newton, leaving Leatherback circling the immobilized Jaeger, hitting its Conn-Pod, sending Herc flying against the wall, breaking his arm. Having no other choice, Pentecost deploys Gipsy Danger due to its analog systems being powered by a nuclear reactor making it immune from EMP attack, saves Striker in time before the kaiju can smash it and its crew. The Jaeger then manages to kill both Kaiju, using its plasma cannon to take down Leatherback and a built-in sword to slice Otachi in half while in the air. Newton and Chau inspect the corpses, find out that Otachi is pregnant. The infant Kaiju bursts out, swallows Chau whole, and dies strangled by its own umbilical cord. Newton and Hermann drift together with the infant’s brain, discover the Breach will only open in the presence of a Kaiju’s DNA.
Shatterdome proceeds with the attack on the Breach with Gipsy Danger and Striker Eureka. Pentecost, who is dying from radiation sickness, replaces the injured Herc Hansen to pilot Striker Eureka with Herc’s son Chuck. Two Category-4, Scunner and Raiju, with one Category-5, Slattern, emerge from the Breach to defend it. Gipsy Danger kills Raiju, but Striker Eureka is immobilized by Slattern. Gipsy Danger attempts to help Striker Eureka but Pentecost warns the crippled Jaeger to stay away, telling them to use Gipsy Danger’s nuclear reactor to destroy the Breach. Pentecost and Chuck sacrifice themselves as they detonate the bomb, killing Scunner while also mortally wounding Slattern.
Gipsy Danger finishes off Slattern with a blast from its nuclear vortex then uses its corpse to go through the Breach, where Raleigh sets Gipsy’s reactor to detonate, before ejecting from Gipsy and being thrown back through the Breach. The reactor detonates, closing the Breach. The two escape pods surface in the Pacific Ocean, while Raleigh and Mako embrace as helicopters are sent out to retrieve them.
At the start of the credits, Chau cuts his way out of the infant Kaiju carcass and asks where his shoe is.