The episode opens with Barry Allen and Iris West at their loft, trying to understand what the Red Death needs the cosmic treadmill for. They still believe Red Death is a man. Meanwhile, Mark Blaine is seen working hard on the cosmic treadmill, visibly troubled and conflicted. Mark's grief over the death of his partner Frost is evident, and he is desperate to regain what he has lost, even if it means betraying his former friends.

Barry visits Mark at his lair and offers him water, telling him he doesn't have to help Red Death. Mark expresses that he thinks it is too late for him, but Barry insists that Mark is still part of Team Flash and that people make mistakes, emphasizing that it is never too late for second chances.

At the West-Allen apartment, Iris is visited by Ryan Wilder, who claims she was injured by the EMP that knocked out Central City's electronics. Iris is suspicious of Ryan's story. Ryan reveals that she accidentally killed her Iris while fighting the Flash, which Iris believes, but she does not trust the rest of Ryan's story. This angers Iris, prompting Ryan to reveal her true intentions: she wants to kill Barry Allen, the Flash. Ryan then calls on her Red Death suit, becomes deranged, and kidnaps Iris.

Ryan uses Iris as bait to force Barry to run on the cosmic treadmill. Barry complies and starts running, which causes lightning to strike throughout the city, including at the West house. Cecile uses her powers to save Jenna and then rushes to help other neighbors affected by the blackout.

At STAR Labs, with Barry restrained by meta cuffs, Team Flash now has a location for him. They plan a rescue using Allegra's teleportation device. Meanwhile, at the lair, Mark recalls Barry's pep talk and decides to sabotage the treadmill, temporarily disabling Ryan's Red Death suit and destroying the device powering it.

With Barry's speed drained, Mark fights the evil Rogues who are working with Red Death to give Barry a chance to escape. The Rogue Squad arrives to assist, but when Red Death begins powering up again, Team Flash decides to leave Mark behind and teleports back to STAR Labs. Mark is left behind, and it appears he may have been killed by Red Death's crew.

Back at STAR Labs, the team regroups and discusses the dire situation. They realize that Red Death's true identity is Ryan Wilder, formerly Batwoman, and that she has been manipulating them. The episode ends with the team preparing for the escalating threat posed by Red Death and her upgraded supervillains, setting the stage for the next part of the story.

At the end of The Flash Season 9, Episode 4, "Mask of the Red Death (1)," Barry Allen (The Flash) is held captive in Red Death's lair after being forced to power up the Cosmic Treadmill to save Iris, who was kidnapped. Mark Blaine, who has betrayed Team Flash by siding with Red Death, has a change of heart, sabotages the treadmill, and injures Red Death, allowing Barry to escape. However, Barry is exhausted and weak. As Team Flash and the Rogue Squad arrive to rescue him, they are confronted by Red Death's minions. Mark distracts the villains to give the team a chance to escape, but Red Death begins to recover. Despite Barry's pleas to help Mark, the team leaves him behind, seemingly to his death, and teleports back to STAR Labs with Barry safe but the threat of Red Death still looming.

Narrated scene-by-scene ending of "Mask of the Red Death (1)":

The episode's climax unfolds inside Red Death's dark lair, where Barry is imprisoned alongside the kidnapped Iris West. Red Death, revealed to be Ryan Wilder (the current Batwoman), commands Mark Blaine to complete the Cosmic Treadmill, a device capable of devastating power. Barry is forced to activate the treadmill to prevent harm from coming to Iris. As Barry's Speed Force energizes the machine, the city plunges into darkness, illuminated only by ominous red lights. The blackout causes chaos across Central City, prompting Cecile to use her powers to protect her daughter Jenna and assist the vulnerable citizens.

Meanwhile, at STAR Labs, Team Flash scrambles to locate Barry. The activation of the treadmill triggers Hartley's locator, revealing Red Death's lair. The team calls upon the Rogue Squad, a group of antiheroes, to aid in the rescue mission. As they prepare to intervene, Mark Blaine, conflicted and remorseful, sabotages the treadmill's controls, injuring Red Death and temporarily halting her plans. This act of defiance allows Barry to break free, though he is visibly drained and weak from the ordeal.

The Rogue Squad teleports Team Flash into the lair, but they are immediately confronted by Red Death's upgraded supervillains. A fierce battle ensues, with the team struggling to fend off the enemies. Mark steps forward, distracting the villains to give Team Flash a chance to escape. Barry, still recovering, pleads with his teammates to help Mark, emphasizing that he deserves a second chance despite his betrayal. However, the team, prioritizing Barry's safety and the larger threat, reluctantly leaves Mark behind as Red Death begins to regain consciousness.

The episode closes with Team Flash and the Rogue Squad teleporting back to STAR Labs, Barry safe but exhausted, and Mark's fate left uncertain, implied to be grim. The looming presence of Red Death and her minions signals that the conflict is far from over, setting the stage for the next episode.

In terms of character fates at this episode's end:

• Barry Allen is physically weakened but rescued and safe at STAR Labs.

• Iris West remains unharmed but was used as bait and is now back with the team.

• Mark Blaine, after betraying Team Flash, redeems himself partially by sabotaging the treadmill but is left behind in the lair, likely facing death.

• Red Death (Ryan Wilder) is injured but not defeated, poised to continue her threat.

• The rest of Team Flash and the Rogue Squad survive the encounter and regroup to plan their next move.

The Flash Season 9, Episode 4 titled "Mask of the Red Death (1)" does not have a specifically documented post-credits scene based on available detailed episode reviews and recaps. The sources discussing this episode focus on the main plot developments, such as the reveal of Red Death's identity as Batwoman Ryan Wilder, Mark Blaine's betrayal, and the escalating conflict with Red Death's upgraded villains, but none mention a post-credits scene for this particular episode.

While The Flash series has included post-credits scenes in previous seasons (notably at the end of Season 8 to set up Season 9), there is no clear evidence or description of a post-credits scene following Season 9, Episode 4 specifically. The available episode reviews and recaps do not note any additional scenes after the credits for this episode.

Therefore, it appears that "Mask of the Red Death (1)" does not feature a post-credits scene.

The Red Death is revealed to be Ryan Wilder, the Batwoman from another Earth, who has upgraded supervillains and is using a remote-controlled suit to fight Barry Allen.

Mark Blaine betrays Team Flash by siding with Red Death due to his desperation after the death of his love, Frost. However, after a pep talk from Barry about second chances, Mark sabotages the treadmill powering Red Death's suit, temporarily disabling it and fighting the evil Rogues to give Barry a chance to escape.

Barry is ambushed by Roy Bivolo, a fear-inducing supervillain allied with Red Death, which causes a citywide blackout. Barry is taken prisoner, locked up in Red Death's hideout with meta-dampening cuffs and chains, and forced to run on the treadmill to power Red Death's suit.

Ryan Wilder approaches Iris in her civilian identity to gauge what she knows. Iris quickly deduces Ryan is the villain Red Death. Ryan reveals she accidentally killed her own Iris while fighting the Flash and tries to manipulate Iris, eventually kidnapping her to use as bait to force Barry to run on the treadmill.

The citywide blackout is caused by Roy Bivolo's attack during Barry's ambush, crippling STAR Labs and Central City. Team Flash calls upon the Rogue Squad to help rescue Barry. Cecile uses her powers to protect neighbors during the blackout, and Allegra uses her teleportation to bring the team back to STAR Labs after the rescue attempt.

The Flash Season 9, Episode 4 ("Mask of the Red Death, Part 1") is generally suitable for a family audience but contains some elements that might be upsetting or objectionable for very young children or sensitive viewers. The episode involves intense situations such as The Flash being chained up, a power outage causing citywide problems, and the presence of a villain (Red Death) who exerts threatening control. There are scenes with tension, suspense, and some emotional distress, including a character who is wounded and seeking help. The tone includes moments of suspicion and confrontation, and there is some implied violence related to the villain's actions, though nothing excessively graphic is reported.

Potentially sensitive aspects include:

• Scenes of captivity and restraint (The Flash chained up).
• A citywide blackout causing chaos and urgency.
• Emotional tension and suspicion between characters.
• Presence of a villain with menacing intentions.
• A wounded character seeking refuge, implying prior violence.
• Some dramatic confrontations and moral dilemmas.

No explicit content, graphic violence, or strong language is noted in reviews, but the episode's darker themes and suspenseful tone might be unsettling for very young children or highly sensitive viewers. Overall, it fits within typical superhero drama standards but with some intense moments that parents might want to be aware of before viewing with younger children.