"The Mob Doctor" episode "Fluid Dynamics" opens with a beautiful woman casing the joint at an auction pre-sale. She uses her phone to take photos of security cameras and flirts with the man in charge of the auction to distract him while she plants a small electronic device underneath a counter.
Meanwhile, at the hospital, preparations are under way for the staff Christmas party. A dark-haired man in his 50s comes in looking for Grace and says that he was sent by a "mutual friend." At first, it just seems like the man is trying to score some painkillers, but then he collapses, and it is clear that this guy is truly ill.
Grace calls Constantine, assuming that he was the "mutual friend" who sent the mysterious man. Constantine knows the man, calling him Russell King, but says that he didn't send Russell to see Grace.
The "mutual friend" was Franco, and by calling Constantine and alerting him to Russell's presence, Grace may have endangered Franco's life. Franco tries to rekindle things with Grace, but she seems reluctant.
Elsewhere, a man named Kyle Bennett shows up at the hospital with pieces of a metal reindeer antler Christmas display sticking out of his head. Dr. White starts acting weird around Kyle, and the rest of the staff starts wondering why. It is later revealed that Dr. White was once Kyle's step-father and has fond memories of taking him to a ball game. The two have been estranged for years, but Dr. White still feels a fatherly love toward the boy, who is now a man.
Later, Russell's daughter (the woman who was casing the jewelry auction at the start of the episode) shows up. She says that she can do the heist on her own if her dad is too sick, and Grace says they need to do more tests. Constantine also stops by Russell's hospital room, telling him that if he's planning a heist in Chicago, he'd better plan on sharing the wealth with Constantine.
Constantine and his associates soon find out what Russell and his daughter plan to steal, and that they plan to get access through the shutdown water mains.
Grace learns that King is suffering from an abdominal aortic aneurism, which could kill him instantly. But it's too late: Russell has left the hospital to do the heist. While he's down in the water main, Constantine convinces Public Works to turn the water back on.
The tunnel floods, but Franco and Russell get out in time. However, Russell's aneurism bursts. Grace is able to get him to the hospital, and she saves his life. Later, Grace shows up at Franco's apartment and kisses him passionately, resuming the relationship they had years ago. Constantine's men, however, know that Grace has been helping Franco, and Constantine will have to deal with her.