In episode four, “The Mask of the Red Death, Part I,” the show centers on how the Flash is forced to make an impossible choice in his fight against the now-revealed red speedster, known as “Red Death.”

The Red Death has launched her attack on Central City and, with the power out in the city, Team Flash recruits its Rogue Squad to help, while Iris is visited by an “old friend.”

The episode begins with a glimpse into the lair of Red Death, where Chillblaine is constructing the cosmic treadmill. Red Death enters the lair and orders Chillblaine to finish the construction sooner. It is revealed that the treadmill still needs a power source, but Red Death tells him to leave that to her. Red Death leaves to recruit yet another rogue to her growing team.

Meanwhile at the loft, Barry and Iris are continuing their investigation into the mysterious Red Death. They still believe that the evil speedster is not only a man but the new avatar for the Negative Speed Force. Barry thinks the negative speed force is corrupting Red Death, seeing it as an opportunity to save her — something he could not do for his longtime foeEobard Thawne, or “Reverse Flash,” in the season eight finale.

The episode shifts to the West household where Joe is reading his youngest daughter Jenna a bedtime story. Cecile walks into the house and they start talking about leaving town.

Meanwhile back at S.T.A.R Labs, Chester is sitting at his computer and talking to one of his viewers about his awkward encounter with Allegra. He tells his follower that he’s giving up his chances with her, but his follower pushes him not to.

Later, Barry arrives and the two of them track down the Red Death and her whereabouts. Red Death launches her attack on Central City by shutting off the electric grid. Red Death is prepared for the Flash, as it is revealed she has recruited Roy Bivolo, an enemy Flash fans have not seen since season one. Due to Bivolo and Red Death’s alliance and Flash’s misjudged attempt to convince her to break her connection to the Negative Speed Force, Flash is kidnapped.

Team Flash realizes that they have no way to find their leader and defeat the evil speedster, so Chester sends out an alert to the other Rogues for help. When the Rogue Squad arrives, they try to track down the Flash, but because he lost his speed, they are incapable of it, which Chester points out and Hartley tells the team about. Allegra is not showing any defense of Chester as she accepts the Rogues pushing him around to keep her distance.

The episode shifts to a scene in the Red Death’s lair where viewers see Barry and the villainess’s remote-controlled armor talk about her plan, while the real Red Death does the same with Iris. She tries to convince Iris to team up with her but fails. In Barry’s conversation, it is revealed that the power source needed for the cosmic treadmill is the Flash himself,while Iris’s conversation with Red Death starts with deception as Ryan Wilder mysteriously shows up at her apartment. Ryan tells Iris her story, but Iris is not buying it. This causes Red Death to get angry and take Iris by force.

Chillblaine and Barry get to talk to each other about Mark’s recent betrayal. He tells Barry not to waste his time trying to get through to him as he’s just a lost cause now. But Barry counters his remarks and tries to talk some sense into Chillblaine. When Red Death returns, this time with Iris, she forces Barry to activate the cosmic treadmill. Barry begins to run,  which causes red lightning to appear. This window of opportunity allows both teams to track Barry and Iris down. Allegra and the Rogues then go to rescue them.

Red lightning strikes and blows glass at Joe and Cecile’s daughter. Cecile uses her powers to save her daughter, then goes off to help others. Meanwhile, Barry is still running on the treadmill, gradually starting to lose energy. Mark realizes his mistake and turns on Red Death. Red Death’s team arrives and fights Mark.

The squad rescues Barry, while Hartley grabs the teleportation bomb from Allegra and uses it to escape with the group. Before they leave, Barry sees that Mercer stabbed Mark, which may mean he’s killed him.

Barry argues with everyone about leaving Mark behind as he wanted to help Mark. Iris reveals what Red Death’s plans are.

Allegra apologizes to Chester for not wanting to talk about their growing relationship. Mark’s death by Mercer reminded her about life’s importance. She wanted to talk about their relationship, but now Chester is the one who doesn’t want to talk about it.

In the end, Joe and Cecile decide they should stay in Central City for their daughter. Red Death organizes a new plan with her Rogues to take out the Flash and his team and take his world as her own.