After she has an adverse reaction to the revelation of Dinah's kidnapping, Coil realizes Skitter is likely to turn on him at some point.
Coil starts taking precautions in anticipation of the Undersiders' attempts at betrayal and sets up to betray Skitter first. When sent to intimidate the Mayor, Trickster carries recording devices for Leet to create the "famine engine", a device that mimics Skitter's power.
At the same time the Undersiders, mainly Skitter and Tattletale, begin planning to defeat Coil. They do not tell the others about the extent of the plan, ensuring that they were able to keep the plan secret.
After Coil successfully faked his death, and his civilian identity, Thomas Calvert, replaced Emily Piggot as the acting Brockton Bay PRT director, the Undersiders met with Calvert to negotiate the release of Dinah, as Calvert had successfully cemented his control over the city. Calvert agreed to relase Dinah to her family, and temporarily released her into Skitter's custody.
Calvert shot Skitter in the chest from close range with a pistol. Taylor thought that it successfully penetrated her costume, but it did not. As Taylor began to react to this using her swarm, Calvert sent men with Molotov cocktails in to set fire to the room and her. Taylor used her pepper spray to set one of the men on fire as he attempted to light his cocktail, but Calvert used the other man's cocktail to feed the flame. Calvert shot the first man, and barricaded the door closed. Calvert had prepared the area as a kill zone for Taylor, bulldozing nearby houses, setting up a chain-link fence perimeter outside it, and boarding up both windows in the room.
Using her gun, which Taylor believed Coil was unaware of and thus had not planned for when preparing the area, Taylor shot out the boards over one of the windows. Meanwhile, Calvert ordered his men to burn the building to the ground, and upon noticing Taylor shooting the boards out, they also fired into the room/window with machine guns.
In response to Calvert's men's actions, Taylor used cockroaches to eat through the headlight wires of the PRT trucks that were illuminating the exterior of the building, while using most of the rest of her swarm to make human silhouette decoys of herself. Thanks to the many decoys and general confusion caused by her swarm, Taylor was able to escape the burning room through the window.
Using her swarm-speech and pulling the soldier's smoke grenade's pins with her swarm, Taylor sowed further confusion among Calvert's men as she approached the chain-link perimeter. Eventually, Calvert left the scene. Taylor successfully exited the perimeter, and stole one of his men's vehicles to find the Undersiders, anticipating that he would now make a move against either the rest of the Undersiders or her father.
Taylor successfully located the Undersiders by using her swarm to find Coil's men that were tracking the Undersiders (along with the body double Calvert had replaced Taylor with when he teleported her out, and minus Tattletale, Regent, and Shatterbird, who Calvert currently was holding hostage) and Dinah. Considering that the men monitoring the Undersiders had at least one mortar, as well as several sniper rifles, Taylor was reluctant to reunite with the Undersiders and reveal her double while the rest of the Undersiders were in mortar range of Calvert's men. Using a ladybug Taylor placed on her hand, Dinah was able to signal to Taylor that her precognitive abilities told her that the most likely route to success would be for Taylor to get inside Calvert's men's perimeter. She did so, using the storm drains, and found that she could now reach each of the four mortar teams that were a part of the cordon with her swarm. She attacked all four teams at once, but the fake Skitter responded to this action (presumably because she had a radio earpiece and Calvert could contact her via it) "betraying" the Undersiders. A "famine engine", which was created by Leet to approximate Taylor's swarm control, was activated, telling all the nearby insects to attack nearby humans indiscriminately. Fake Skitter then takes Dinah hostage, shoots at Bitch, flashbangs the Undersiders, and is teleported out via Chariot's teleporter. Taylor used her swarm sense to determine the location of the famine engine, and moves to destroy it, to prove to the Undersiders that they were betrayed by a body double. The rest of the Undersiders catch up with her before she can destroy it and prepare to execute her, but she convinces Grue to activate his power, and he is able to tell via it and the borrowed swarm-sense that she grants that she is not controlling the swarm. Grue gets Bitch to use her dogs to destroy the famine engine.
The Undersiders managed to convince Calvert that they killed the real Skitter after the fake Skitter betrayed them, and tell him they know that fake Skitter escaped using Chariot's teleporter, which they know Calvert as control over. They told him that they want to meet him in person at the Brockton Bay market. At the meeting, they presented Coil with a false corpse of Skitter. Coil called them on their bluff, as thanks to him having access to the resources of the PRT in addition to those of his own organization, he was able to determine that they were lying about Skitter's death. He teleported in a large team of his soldiers, the Travelers, Uber and Leet, Chariot, and Circus into the market.
Taylor is able to prevent Calvert from executing the Undersiders immediately by claiming that Charlotte will email the PRT and others with the details of who Thomas Calvert really is, unless Taylor checks in with her regularly. She convinced Calvert to prove to Taylor that Tattletale is alright. A restrained Tattletale body double is teleported in, and Calvert refuses to let Taylor confirm that the double really is Tattletale. As such, she called him on his bluff, and convinced him to teleport in the real Tattletale. When Tattletale used their code to confirm her identity, she uses the word "stringbean" as a code phrase for Calvert's men. Moments later, when Calvert ordered the Undersiders' execution, several shots are fired, but half of Calvert's men are holding the other half at gunpoint and preventing the execution. Some of Calvert's men freed Tattletale, and she explained that over the course of working with Coil, she stole extra valuables from their targets (one example she provided is that during the bank heist she used the CEO's terminal to transfer funds to an account of her own), and when Dragon sent her suits to Brockton Bay, she convinced Coil to spend a great deal of his wealth to get information from the Dragonslayers on how to beat her. As such, she now has access to more money than Calvert does, and she set up the code phrase with some of Calvert's lieutenants. Calvert is furious and does his best to regain control of his men, to no avail, and Taylor and Lisa managed to get him to activate his power after Tattletale used her code phrase, meaning that he cannot use his power to escape the situation. He attempts to get the Travelers to stop the Undersiders, but Taylor convinces Ballistic to side with the Undersiders, and reminds the remaining Travelers that unlike the Undersiders, the Travelers do not have bulletproof costumes. Tattletale tells the Travelers that Calvert has no viable plan to help Noelle, meaning his method of control over them has no weight. Before the remaining Travelers can move against the Undersiders, Imp nonlethally disables all of them. After Calvert spits some final insults at Lisa and Taylor, and pretends to admit defeat by saying he's ready to go to the Birdcage (Tattletale is confident Calvert has contingency plans in place to free him if they tried to send him to the Birdcage, so they reject that idea), Taylor has a moment of reflection, and executes Calvert.
The Undersiders subverted and absorbed all of Thomas Calvert's wealth and connections.
The Undersiders go to the underground base to check on their still captured teammates and dismiss unwanted employees, only to find that Thomas is spiteful in death.
The Travellers secret was set loose on the recovering Brockton Bay, Skitter is already in very bad shape from everything that's happened and now she has to deal with what might well be a nascent Endbringer.