It has been 6 month since the events of the season 3 finale. Barry is in the speed force and Iris doing her best at moving on as per his wishes. Part of that moving on is Iris now the leader of Team Flash as she coordinates Kid Flash and Vibe in catching an escaped Peek-a-boo.
With Iris the leader, she is all-business and gets Wally to do some more training to increase his speed making him miss the family dinner. At this family dinner, we learn that Cecile has moved in with Joe. We also learn that Julian has moved back to London and Cecile is looking to replace Barry. Joe wants to be sure that Iris is okay with. On the surface Iris says she is okay as they all have to move on. But Joe knows something is up when Iris ends the evening early to return home. Even at home her existence is a lonely one and we see that she is sleeping on the couch. But there is no rest for the weary as there is an alert to an attack on Central City.
Back at STAR Labs, Cisco suggests bringing back Barry from the Speed Force but everyone thinks it is a bad idea as no one wants to unleash another speed storm. However, Cisco has been working with Harry, Tina, Tracy, Felicity and Curtis on a way to bring Barry back but also keep the speed force happy. Joe and Wally is on board with Cisco’s idea but Iris is not. This surprises Cisco as he thought she would be on board with bringing the love of her life back.
Cisco decides to move forward in bringing Barry back and goes to a dive bar to enlist Caitlin’s help. Caitlin is back to her normal self with no trace of Killer Frost but has abandoned her career to work as a bartender. Cisco implores Caitlin to help them and she eventually agrees. During their exchange, their talk was interrupted by some one-eyed tough guy that clearly is trying to keep Caitlin under foot.
The next day, in the empty airfield where Team Flash first tested Barry’s speed, the team (minus Iris) has assembled. In fact, Iris gets wind of it when no one was at STAR Labs. When Joe sees Caitlin, she quickly apologizes and asks for his forgiveness for what she did to him and Cecile. Joe accepts her apology and forgives her.
Part of the plan is to use the speed force bazooka and a speed quartz that contains a bit of Barry’s DNA to trick the speed force into thinking Barry is still in it. The sphere quartz disappears but no Barry appears.
Team Flash thinks they have failed but we see Barry emerge naked from the Speed Force 300 miles away from Central City.
Cecile lets the team know that Barry is back. However when the team arrives at the CCPD they find a bearded Barry scribbling symbols on the wall and spouting nonsense.
When Barry is brought to STAR Labs, Caitlin there could be two things wrong with Barry 1) he is schizo psychic or 2) Barry has been in the speed force for 10,000 years and is suffering from dementia.
When the slowly wake up Barry he is still spouting nonsense and writing the symbols. Cisco is determined to translate the symbols that Barry has been writing. Iris then talks to Barry alone and tells how she tried to move on but wants him to come back to her. We see the speed force flash in Barry’s eyes and he starts to speed around STAR Labs. Cisco tries to stop him but it ends being Caitlin that stops him not with her cold powers but with Captain Cold’s gun.
The team put Barry’s into the pipeline for his own safety but are concerned about Samurai’s warning, which he seems to come early. The team gives him the Flash in the form of Wally but the Samurai isn’t fooled and demands the Flash or Central City will fall.
Wally is down for the count as Samurai stabbed his leg pretty bad. Meanwhile, Cisco has a break through with the translation thanks to Caitlin mentioning how the symbols where Greek to her. But after the symbols are translated “This House is Bitchin'” that it makes even less sense.
Joe visits Barry in the pipeline and shaves off his beard thinking it would make him feel better. Joe then talks to Iris about having faith which gives people strength. This gives Iris an idea and tells Barry to come find her.
As Samurai’s deadline ends, Joe and the CCPD are in a showdown. It ends with Iris telling Samurai to take her as she knows the Flash will come after her. Samurai flies off with Iris and Joe rushes to Barry to tell him that Iris has been taken. At first Barry continues to write his symbols but at Joe’s further pleading to save Iris this causes him to break free, grab his new suit and save Iris.
The chase ends in a field with wind turbines and Barry saving Iris and knocking down Samurai. When Barry unmasks Samurai, it ends up being a robot. Barry and Iris are reunited and share a kiss.
At STAR Labs, Caitlin gives him a clean bill of health and thinks being in the Speed Force has helped increase his speed. Barry doesn’t know if that is true or not as he can’t recall his time in there only when he went in and when he saved Iris. Cisco dubs the samurai Samuroid and is trying to figure out who sent it.
Team Flash is back and this includes Caitlin. Caitlin goes to quit her job at the bar but when that tough guy says the boss won’t be happy, she doesn’t car. The tough guy tries to strong arm Caitlin but this causes her to turn back into Killer Frost and she ends up threatening him. Killer Frost is ready to leave the bar for some fun but stops as she then turns back into Caitlin. This creates a Jekyll and Hyde moment for Caitlin.
Iris and Barry return home and he sees how much Iris has been struggling without him. Barry promises never to leave her again but gets called away for Flash duties.
In the coda, we see the Samuroid being put back together and we see a woman looking at a news article proclaiming the Flash is back. It looks like we are in the future. The woman tells an unseen man that their plan has worked and what is next. The man turns around in his chair and it is The Thinker (who looks like one of the Borg) and tells the woman he is thinking.