The agency now works for the Beaurea of Indian Affairs. Their new sponsor has them investigate some strange activities on a reservation. They soon encounter an invisible creature. Secrets of the gland are revealed.
Recalling how the previous invisible man's DNA was able to take over Darien when he was asleep. Darien asks Claire to find some samples of his brother Kevin, so he can remove the gland from Darien's head. Arnaud, permanently invisible, captures Darien/Kevin to have him fix his gland.
The Invisible Man goes undercover to infiltrate a gang of thieves who supply bomb parts to a dangerous terrorist. Darien is arrested and sent to prison with one of criminals in order to discover the terrorist's next move. Meanwhile, Hobbes and Monroe try to discover the location of a possible EMP device that could cripple modern civilization. The timer is ticking...
After wrenching his back, Darien visits an acupuncturist, in spite of The Official. Dr. Jay, however, accidentally triggers the Quicksilver gland and forces Darien to help her steal a Chinese artifact to save her dying mentor.
Darien cuts himself, and develops an infection. The bacteria from the infection are rendered invisible, believing they can mass produce the bacteria, they withhold treating Darien, but soon many people in the agency are affected. Worse since they withheld treatment for so long, the bacteria are now resistant to conventional treatment, so the infected people will go into Quicksilver madness /death. They race to find the cure.
A person with a knack for selling secrets captures Claire, and injects her with a truth serum. It makes it impossible for her to keep quiet, or lie. She reveals several secrets, including the I-Man project. The truth serum will eventually kill Claire, Darrien and Hobbes race to find her.
Claire disobeys the Official and gives Darien one last injection that fixes the gland, no more quicksilver madness. Darien goes to work for the FBI, and finds out about a plot by Crysallis. The FBI doesn't take him seriously, he needs Claire, and the Agency to stop Cryallis. The end of the series.