Pete and Myka have traced two cases of a strange rusting disease to a factory run by a shady character named Siskel, who quickly becomes their prime suspect. But when a third victim is reported, their investigation takes them to Tully's Boxing Gym, the only location everyone had in common. Pete attempts to soften his questions by asking them in a hard place: the boxing ring. But when he points his suspicion at the mill's union rep, a young boxer named Cody steps in and knocks his lights out. Myka sees a strange shadow flicker across his arm as he does so.
The agents speak to Cody's father, whom he saved from a horrible accident at the factory - an incident that they believe Siskel set up for the insurance money. They also learn that, years earlier, Cody saved two of his fellow Marines in a gruesome explosion in a museum housing artifacts from the stand that 300 Spartan soldiers took against the Persians. In this explosion, Cody was hit with shrapnel, and Artie believes that the shard of artifact lodged in his body activated itself when Cody saved his father. Now, anyone that Cody touches while in an emotional state gets consumed by rust.
Cody overhears the conversation between Artie and his agents, and, horrified at having made so many people deathly ill, runs to the factory to revenge himself on Siskel. Pete catches up to Cody, who batters him with blows and gives him the rusting disease, but Pete is finally able to convince Cody not to go after Siskel - Cody's not a killer. Myka catches up after extracting a promise to rehire any workers he laid off and keep them in safe working conditions, but since the artifact is in Cody's body, she has to inject the goo into his chest. Cody's heart glows purple, and Pete and the other victims slowly return to normal.
Jinks and Claudia have been trying to find a way to free Jinks from the metronome, and their discovery of a poem regarding it leads them to Steve's childhood home. He and his mother, Emma, haven't spoken since the sentencing of the killer of Steve's sister - Steve wanted the death penalty for the man while his mother sought compassion and a lighter sentence. Steve and Claudia hoped that returning home would satisfy the poem's line "make a pure start from whence one comes," but when they stop the metronome, they nearly kill themselves both.
Jinks has a huge fight with his mom, and later, when all three reconcile and accidentally place hands on the metronome, Emma's heart stops. In a moment of sacrifice, Jinks shatters the metronome. A flash of light appears, his mother is safe, and Jinks is still alive. His act of pure love saved his mother, Claudia, and himself.
Back at the Warehouse, Artie agrees with Mrs. Frederic (brought into the discussion on Magellan's astrolabe by H.G. Wells) that Father Adrian tried to isolate Artie and keep him weak by urging him not to tell anyone of his use of the instrument. Then, without including Artie, Mrs. Frederic helps H.G. secret the astrolabe away - the less Artie knows of its whereabouts, the better - and asks her to keep radio silence with the Warehouse, and to trust no one.