On a dark and deserted road, two robbers arrive at an abandoned building on a mission to steal valuable copper. But it looks like someone beat them to the punch; the padlock has been cut open with what looks like a welding torch. They continue to move through and approach the cargo door with another deformed latch. They consider turning back, but Stu is determined to get his hands on that copper. They enter the boiler room and discover that nearly all the copper plumbing has already been removed.
They hear a noise; the other robber must still be here! The two robbers move towards the sound of coughing and are met by a blast of fine spray, illuminating the mist with their flashlight. The raspy cough continues and is followed by a small burst of flame that erupts in a huge explosion when it comes in contact with the strange mist. The fireball throws Stu and Joe back against the wall, torching them on contact. Later, Nick and Hank arrive on the scene and evaluate the damage. The extent to which the bodies were burned indicates that gasoline wasn't the culprit; this was like getting hit by napalm. After evaluating the substance, they find that it's human fat mixed with methane.
A witness comes forward, explaining the man he believes to be the culprit. He was an old man who looked homeless, and half of his face was seriously scarred up. Nick and Hank are interrupted by a call; apparently, they've got a report of another break-in back at the scene of the crime.
Wu, Hank and Nick cautiously enter the premises and encounter the same old homeless man that the witness described. Nick sees the man morph into a frightening, scaly dragon-creature called a Daemonfeuer. But they act too late; he sprays them with the strange mist, sending the three into a state of delirium. They dive aside just as the mist ignites into a wall of flame. In the midst of the commotion, the man gets away, but they were able to get an I.D. on the lipids. They belong to Fred Eberhart, a veteran and welder.
Fred has a daughter, Ariel, who's their best bet for tracking him down. Nick decides to intercept Ariel at the Club Trop Chaud, her place of work. But when he arrives at the club, Nick's surprised to find that Ariel is a fire dancer, transforming into a Daemonfeuer during her performance. She flirtatiously makes eyes at Nick, intrigued by the presence of a Grimm.
Nick notices that there are an unusual number of Grimm creatures here... and even spots Monroe! He goes over to talk to Monroe, asking him about the Daemonfeuer. In addition to their fire-breathing ways, they're treasure-mongers and love to plunder and collect. They also value the old dragon-like traditions from the days of yore... knights in shining armor, the whole nine yards.
Nick questions Ariel after her show, who makes unwelcome advances towards a visibly uncomfortable Nick. Ariel claims she has no idea where her father is. After her mother died, he fell apart and blamed himself for her death. He couldn't deal with anything after that, specifically her. Nick's still suspicious and tails Ariel home, only to have her pounce on him and tackle him to the floor, ripping open his shirt and planting a kiss on him. Clearly, this woman is a handful.
Despite her antics, it turns out Nick had good reason to be suspicious of Ariel. Later that night, she visits a cave hidden deep in the mountains, where her visibly ill father lies. She promises him that it's going to be okay... she's found someone who's going to make him better. What does Ariel have in store for Nick?
The next day, Ariel calls Nick, telling him to come by her house tonight. She promises that she's done playing games and will tell him everything she knows to help him find her father. Nick doesn't make the same mistake and arrives with Hank this time. They find that her copper "lair" is empty; it looks like she must have bailed.
Nick arrives home and heads to bed, only to find that Ariel is in his bed instead of Juliette! The question is... what has she done with Juliette? Nick throws Ariel against the wall, demanding to know what she's done. He chases her downstairs, but she's already gone, with no car anywhere in sight. He gets a call from Ariel on Juliette's phone, telling Nick it's time for a little search and rescue. We see a terrified Juliette, bounded up in the back of her car.
Ariel continues, telling Nick that worthy adversaries are impossible to find, and she'd lost all hope of being able to help her father. That is, until she found Nick. Ariel tells Nick that it's time to Grimm up and face his Daemonfeuer, as she drags Juliette deep into the mountains.
Nick recruits Monroe on his mission, and the two quickly hit the road. Monroe explains that Nick's princess has been taken by the dragon, and it's Nick job to attempt a rescue. The only problem is that, in this scenario, someone has to die. Nick and Monroe deduce that if they follow the old train tracks, they lead to a tunnel, which is the perfect place for the lair.
Their guess is right, and Nick and Monroe cautiously approach the lair. They sneak off in different directions, as Monroe tracks down Juliette and is able to break her free, while Nick goes off to "fight the dragon." It's a tough battle with the old man, and Nick must duck and dive to avoid his deadly flames. He eventually is able to "slay the dragon," stabbing him through the heart with a copper pole. Ariel then arrives on the scene; while she's sad to see her father dead, she's proud that he went out valiantly. Nick sprints out of the cave as Ariel sprays her lipid mist, enveloping the cave in a giant fireball.
Nick makes it out just in time, as the lair explodes with Ariel inside. While they're all safe and sound, Nick definitely has some explaining to do. Juliette is clearly upset and tells Nick that she's not sure if she can tolerate the continual stress that his job puts on their everyday lives. She doesn't want to make decisions tonight, but their future together is definitely something they need to face.