The Mecha-Fleas are the secondary antagonists of "O.W.C.A. Files". They are a group of robotic fleas created and designed by the infamous Professor Parenthesis to help him take over O.W.C.A. by taking control of all of its animal agents to do his bidding for him.
They first appeared when all of O.W.C.A.'s animal agents started itching (even wearing specialized flea collars). Carl spotted one of the tiny Mecha-Fleas and explained this to Monogram, who wonders what kind of freak would build tiny robotic fleas. It was then Professor Parenthesis appeared on a hologram, explaining that he was the one who put the Mecha-Fleas inside the collars so that they can latch on to every animal agent's neural system as part of his plan to take over O.W.C.A.. With a specialized remote, Professor Parenthesis activates the tiny Mecha-Fleas to take control all of O.W.C.A.'s agents (except for Perry and his team), forcing Monogram and Carl to hide away.
A group of giant Mecha-Fleas appear when Perry and his team (including the former evil scientist Heinz Doofenshmirtz) fly through the Grand Canyon (where Professor Parenthesis's lair is located at). They started attacking the agents, who then fight back. However, the agents are forced to jump into the canyon water to evade their hovercraft, which then crashes into one side of the canyon, destroying several of the Mecha-Fleas. Despite the setback, Parenthesis was able to confine Perry and his team in a death trap and heads off with his four remaining Mecha-Fleas (all of them named Susan, Jim, Tracy, and Connie) to O.W.C.A.'s headquarters to finalize his plan. Fortunately, Doofenshmirtz (who remain outside the lair during the agents' infiltration) was able to rescue the agents before Parenthesis' lair exploded.
Meanwhile, Monogram and Carl are finally caught by the mind-controlled agents in their failed attempt to escape, just as Parenthesis and his giant Mecha-Fleas arrive to the scene. As Parenthesis gloats over the success of his plan, he states that once the Mecha-Fleas completely override the agents' natural instincts in 6 minutes, he'll be using the opportunity to unleash the mind-controlled agents to wreak havoc across the world to smear the good name of O.W.C.A.. However, Perry instantly arrives and beats down the giant Mecha-Fleas before kicking Parenthesis in the face. However, fate takes a turn when Parenthesis turns out to be an android being controlled by his true form: that of a tiny blue flea. Despite the ambush, Parenthesis was still able to get the mind-controlled agents to take Monogram and Carl hostage while he and the giant Mecha-Fleas pursue Perry. Fortunately, Doofenshmirtz and the rest of the agent trainees manage to set up a trap for the parasites (per Perry's instructions), and Karen the Cat manage to destroy the giant Mecha-Fleas before engaging into a fight with Parenthesis. Despite the loss of his giant Mecha-Fleas, Parenthesis gloats that his control over the agents will be permanent in 5 more seconds thanks to his remote. However, a household fan falls on Parenthesis' android, thanks to some talking bugs, who were using the fan to fly their paper plane up to the top of O.W.C.A.'s headquarters to enlist themselves as new recruits throughout the entire special. This causes both Parenthesis' android and the remote to be destroyed, permanently disabling all the remaining Mecha-Fleas and freeing all the animal agents from Parenthesis' control. Following the Mecha-Fleas' destruction, Doofenshmirtz took the opportunity to capture Parenthesis with a pair of tweezers and confine him in an enclosed pill bottle for life as punishment for his crimes.