The Undersiders' boss offered a big payday to the Undersiders to embarrass the heroes and PRT at a fundraiser. With only a few hours to plan the group agrees to hit the art gallery. They are not explicitly told to steal anything, the intent is to "shame" the local Protectorate team, and disrupt the narrative that they played a major role in pacifying the Azn Bad Boys.
Initially catching the heroes off guard and using a combination of Grue's darkness and stolen containment foam to disable the PRT agents, underage heroes, Triumph and Assault and Battery, the Undersiders are captured by the remaining Protectorate.
Armsmaster downs Tattletale when she uses her power to read him. The Undersiders talk Armsmaster into ordering Miss Militia to switch weapons, serving as a distraction. Regent keeps Miss Militia incapacitated with his power; Grue and Bitch engage Armsmaster, while Skitter defeats Velocity. Armsmaster incapacitates Grue and two of Bitch's dogs. Tattletale recovers and attempts to use a gun, but is once again taken down by Armsmaster.
Skitter quietly attempts to bargain Armsmaster into letting them go, then swarms him when he refuses, using her body to disable his halberd. She signals for the rest of the Undersiders to leave, which they do, bumping him and allowing her to steal his halberd and throw it off the side of the building as they leave. Skitter deliberately leaves her message to the Undersiders - LETS GO - in place as a final "fuck you" to Armsmaster.