- "Any preference on how you'd like to die? The flame or the frost?"
- —Leonard Snart threatens The Flash
|Leonard Snart AKA Captain Cold returns to Central City with the Cold Gun and a new hotheaded partner in town – Mick Rory AKA Heat Wave. The duo plan to steal a multimillion-dollar painting, but Cold has another agenda while in town – to set a trap for The Flash. Snart and Mick kidnap someone close to The Flash and threaten to kill them unless The Flash shows up for a battle of fire and ice. Barry tells Joe he isn't sure he should take on Snart again after the casualties that happened the last time they fought. Meanwhile, Barry asks Dr. Wells, Caitlin and Cisco to help him double his training efforts so he's ready for the Reverse-Flash when the man in yellow returns to Central City. Iris deals with the aftermath of Barry's confession, and Cisco makes the CCPD a new shield.|
- Grant Gustin as Barry Allen/The Flash
- Candice Patton as Iris West
- Danielle Panabaker as Dr. Caitlin Snow
- Rick Cosnett as Detective Eddie Thawne
- Carlos Valdes as Cisco Ramon
- Tom Cavanagh as Dr. "Harrison Wells"
- Jesse L. Martin as Detective Joe West
Special guest stars
- Jason Bell as Cop #2
- Fulvio Cecere as Officer Vukuvich
- Jane Craven as Rachel Rathaway
- Graeme Doyle as Officer Gibbons
- Daniel Mallinson as Cop #4
- Jane Mitsoula as Cop #5
- James Ralph as Osgood Rathaway
- Michael Karl Richards as Cop #1
- Lee Shorten as Executive Staffer
Barry discovers his plan, and agrees with Dr. Wells to not engage Snart in the hope that he goes away and no one gets hurt like the last time.
When the Flash fails to show, Snart and Rory, who now has a gun that can emit absolute hot temperatures, kidnap Caitlin to force Barry out of hiding. Captain Cold freezes Caitlin's car closed when they kidnap her.
Cisco and Barry realize that if they can get Snart and Rory to cross the streams of their respective weapons it will cancel the two guns out. Snart and Rory challenge The Flash to meet them for a battle of fire and ice in the city and to show the world he's real or they'd kill Caitlin. After multiple failed attempts, Barry eventually gets them to cross the streams with Eddie's help, successfully damaging the weapons and disabling the criminal pair. While in transport to Iron Heights Prison, Snart and Rory are broken out by Snart's sister. Meanwhile, Caitlin investigates the cause of Ronnie's transformation before being kidnapped.
- During the training session, when Barry appears to be stunned, Dr. Wells tentatively puts out his left foot as if rising from his wheelchair, then relaxes when it's clear Barry is okay. This foreshadowing his true identity as the speedster Eobard Thawne.
- When preparing for Barry's rematch with Snart and Rory, Cisco and Barry compare the plan to disable their weapons by tricking them into firing at each other to the concept of "crossing the streams" from the film Ghostbusters, with Cisco noting that the film was surprisingly scientifically accurate.
- When opening up his "Nerd Survival Pack" that Iris found, one of the comic books Barry pulls out is "Space Ghost".
- It appears the feedback from the Heat and Cold guns affected Mick and Snart's vocal cords. When they return, Mick sounds gruffer and Snart sounds more nasal.
- When packing, Iris finds her McSnurtle the Turtle doll. In the comics, Merton McSnurtle's secret identity is The Terrific Whatzit, a funny animal version of the Golden Age Flash.
- In the DC comics story line, Jason Rusch becomes the next Firestorm, succeeding Ronnie Raymond.
- When the Rathaways get off the plane, a staffer tells Mr. Rathaway that his son, Hartley, has been calling. Hartley Rathaway is the villain known as the Pied Piper, who will be featured in the next episode ("")
- Near the beginning when Captain Cold freezes the door of the building with all the cars in it, the frost is still spreading when the car crashes through the door, but when Cold exits the car, his window is up. He wouldn't be able to freeze the door if his gun was inside the car with the window up.
- When Cold freezes Caitlin's car, he brings its temperature down to or towards absolute zero (-459.67°F). Caitlin grabs the handle of her car door, but her bare hand doesn't stick to it like dry flesh would against any metal that reaches -80°F or lower.
- When Heat Wave is brought into the police station, he's handcuffed with hands in front, which police shouldn't do; however, when the cut changes to a shot of the police drawing guns on Heat Wave, then back to him, he is handcuffed with his hands behind him.
- At the end, Eddie gives both the Heat gun and Cold gun to Cisco to take care of, saying "We won't be needing these at the trial". The weapons would absolutely need to be given as evidence in the trial; even if somehow they were not required or a trial was not forthcoming, the weapons would still need to be tagged and stored properly as evidence of the crimes. The chain of evidence is not a procedure that is well-portrayed throughout this series.
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