After Constantine learns the Amato brothers are behind on their gaming machine deliveries, Celeste asks why he gave half the downtown action to her and half to Trapani. Constantine says it's because Chicago hasn't been under total mob control since Al Capone, and he suggests sharing the champagne she sent earlier. But Celeste didn't send it, and Constantine discovers that removing the bottle would trigger a bomb, which he defuses. Celeste has dirt on a bomb squad officer who can determine who made the device.
Meanwhile, the son of a U.S. attorney who was injured in a bicycling accident requests Brett's services. But he's also Brett's brother, Jack, so Brett's called off the case.
Nate summons Grace 'cause he and Victor hijacked a mobster's prize-winning racehorse to steal its sperm. When the horse wouldn't perform, Victor gave it Viagra. Now they have a big problem. Grace gets Ro to deliver an anti-erection drug and feeds it to the horse. Meanwhile, Brett meets Jack's coworker, Carol, and tells her to contact Jack's father, Mark, and Jack's wife. Titus Amato promises to deliver Constantine's machines by week's end.
Back at the hospital, Brett tells Grace that he hasn't seen Jack since their mother died 'cause - unlike Jack - he doesn't get along with their father. When Stafford and Grace operate on Jack, partial paralysis sets in. The doctors need to know exactly what happened during Jack's accident, so Mark orders the surveillance tapes from outside the courthouse delivered within minutes.
Celeste tells Constantine the bomb resembles one used in an insurance scam bombing two years back. Stavos was involved in this and says Jimmy Needles was the bomb-builder.
When the doctors watch the tapes, Grace notices Jack lost consciousness before the accident and theorizes he'd been exposed to a foreign toxin. Nate again summons Grace 'cause the horse is freaking out. Grace wants to tranquilize the horse, but the horse is set to race tomorrow and they can't risk it failing a drug test, so Grace will use ice to reduce the animal's swelling.
Constantine beats Needles into revealing that the Amatos requested the bomb. When Nate takes a picture of Victor icing the horse's member, Victor threatens to kill him and Grace, who defuses the situation, and the now-relieved horse is returned to its owner. Nate apologizes; Grace is furious.
After Grace discovers a swelling on Jack's chest, a black light examination reveals an ultraviolet tattoo that reads "Carol." Brett questions Carol, who admits to having an affair with Jack, who got the tattoo several days ago in Barbados, when they were supposed to be in Toledo.
Constantine and his thugs confront the Amatos. After Titus explains he gave the rights to the machines to Constantine - even if Constantine was dead - two weeks ago, Constantine kills Dimitri, but lets Titus live.
Once tests confirm Brett's suspicion that Jack contracted botulism from a dirty needle, and Jack is cured; Brett tells Grace his anger stems from Mark's asking their mother to forgo chemotherapy until his current appointment was confirmed. When she did this, the cancer spread and killed her. After Brett lies to the family about the source of Jack's infection, Jack expresses his appreciation and admiration for Brett's ability to break away from their father.
Meanwhile, Constantine tells Celeste that Dimitri plotted to kill him and proposes a champagne toast, but gets interrupted by the appearance of a stranger - who turns out to be Constantine's son!