In a moment of desperation, Barry decides to go back in time to save his mother. However, he gets stuck in an alternate universe where he meets a different version of himself. General Zod is threatening to destroy Earth, and there is no Superman to save the day. Barry seeks help from the now old and almost retired Keaton's Batman and mysteriously captivated Kryptonian Supergirl.
The narrative of The Flash tells a more human story for Barry Allen, who is introduced as a lonely person dabbling between personal failures and being a superhero. Barry's character progression as he meets himself in an alternate universe is clinical and organic. The writing for Barry's character seems crisp and clear. He is driven by his desire to find solutions.
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However, it is difficult to look past Ezra Miller's real-life assault case and proceedings, which makes watching him onscreen exhausting. Keaton as the now recluse Batman is subdued. The VFX in many parts of the movie seems rushed and is quite average. The use of technology and scientific intelligence as part of the narrative seems weaker when compared to the recently released superhero movies.
The film's screenplay is tailored well to make you laugh and be consumed by slapstick comedy, action, and electrifying energy. However, it loses steam in the final stretch. If The Flash was not crafted for the multiverse crossover, perhaps it would have come across as a better film.
The cameos in the movie are incorporated to indulge in fan service and evoke nostalgia, but in The Flash, the purpose for the same is unclear. It feels like a fragile attempt to invoke nostalgia, but all it does is make you realize that, just like The Flash, there is not much fixing that can be done for the present DC multiverse.
In conclusion, The Flash is partly fun and partly a lost cause, with Barry making the best of what he can with the movie. The movie also phases the end of Zack Snyder's DC Extended Universe (barring the upcoming Aquaman and the Lost Kingdom).