Syd follows a DSR signal, which leads to a severed arm. Suddenly, a CIA agent named Brodien emerges from the shadows and tells them that his DSR team was ambushed by the infected mob, which has left thousands dead. He reminds them that the Rambaldi device is broadcasting a sub audible frequency that physiologically alters people who drink the water. The symptoms, he says, are irreversible.

He also says the epicentre is located over the Oransky building, and that while he has taken out the city's power grid, the device is still running. Irina suggests that it has been modified to run through an internal energy source. They decide to take the subway as it is the quickest way to the building.

Elena learns of the APO team's arrival. When Sloane volunteers to lead the assault team, Elena tells him to bring Nadia back and give her the option to join them.

As the APO team approaches the subway tunnel, an infected man impales Brodien with a rod. As Jack, Vaughn and Irina try to start the train's emergency battery system, Syd and Nadia go inside the train's switching room to force the train to stop at the Oransky building. Irina urges Vaughn to tell Syd "the truth." Syd tells Nadia that Vaughn asked her to marry him. Nadia and Sydney split up to manually adjust the switches.

Sydney returns to the train first while Nadia, down the tracks, flips the switch. The train rolls down the tracks as a mob chases Nadia. Jack refuses to stop the train because if he does, it won't start again. Sydney stretches to pull Nadia to the train but she's tackled by a madman. Irina tells Syd that Nadia is still alive.

Sloane kills Elena's men and waits at the station. He tells them that Elena has primed the world's drinking water. As Syd voices her suspicions, Sloane says he allied himself with Elena to put an end to this himself, but Elena built in a security device that Sloane couldn't crack. Jack punches Sloane and takes control of the team.

Marshall reports that Elena is going to use the Russian satellite to distribute the Rambaldi frequency worldwide. Once it goes online, the Rambaldi signal will be broadcast and everyone will be infected. Irina says they need to shut down the device from the roof of the building.

Jack tells Sloane to take them to Elena. Sloane cautions that once they disable the device, the toxins will dissipate in the atmosphere and the person who disarms it will only have a few seconds to run. When Irina insists that she can cut the wire and reverse ionisation, Sloane reveals that Elena had changed the wiring scheme as a fail-safe. Irina orders him to take her to Elena so she can force her to reveal information while Syd disarms the bomb.

Nadia fends off the mob and wanders the streets. She approaches a wounded man who surprisingly holds a gun to her head and takes her to Elena. When she refuses to join Elena, Elena injects her with the contaminated water.

Before Syd heads out to the rooftop to disarm the bomb, she turns to Vaughn and says, "Just in case: yes, I'm gonna marry you." She wears his ring and hops into the elevator.

Marshall tries to connect to the satellite. Weiss says the President and his Cabinet are being evacuated to secret bunkers.

Sloane arrives with Vaughn, Irina and Jack. Vaughn searches for the relay room. They strap Elena to a chair.

Sydney reaches the rooftop and opens a hatch that revealed hundreds of interwoven wires. Nadia, now infected by the device, walks toward Sydney. Jack and Irina urge a reluctant Syd to take her out. The sisters battle. Vaughn blows the relay but nothing happens. Irina tells him to get to the roof. Sydney knocks Nadia out and chains her up.

Vaughn arrives on the roof and realizes that Sloane is not there. Meanwhile, Irina guides Sydney in disarming the device. Irina gives her sister a chance to help them as Jack gets ready to torture Elena. When Jack pierces her with a syringe, Elena yells that Sydney should cut the white wire. Irina shoots Elena in the head and tells Syd to cut the blue wire instead and run fast -- she only has 15 seconds.

Sydney is about to cut the wire when Nadia strangles her from behind with the chain. As Sydney starts to suffocate, Sloane arrives and shoots Nadia. Syd sees Vaughn and tells him to take the wounded Nadia with them. Syd then cuts the blue wire and runs away as the device starts to fall apart. They race inside the quaking building and sprints toward a shelter -- they have just saved the world.

Jack advises Irina, who is wanted by the CIA, to disappear. They kiss. Irina tells Syd that she's proud of her. She says that Syd may not see her on her wedding day, but she'll see Syd.

Later, Syd visits Sloane in a jail cell, tells him that Nadia is stable, but they're still trying to find an antidote. The Russian military executed all survivors to support their claim that it was a chemical explosion. Sydney has arranged for Sloane to see Nadia. She believes he was trying to do the right thing. Sloane thanks her.

Driving in Santa Barbara, Syd suggests to Vaughn that they blow off the big wedding and elope. After professing their love, Vaughn reveals that that he was a bad guy before they met. He continues to say that to begin with, his name is not Michael Vaughn. Before Sydney can react, a car collides with theirs.