The video's creator, UltraSargent describes the premise as "What if Days of Future Past was combined with Civil War, with DC and Marvel involved."
The fan-edit features heroes of the Marvel Cinematic Universe, DC Extended Universe, Arrowverse, X-Men movies universe, and Watchmen. With impressive editing techniques, Ultra has managed to amalgamate these TV shows, movies and trailers into quite the spectacle.
This is of course, not the first fan-edit UltraSargent has created; Visit his channel to see his other 50 fan-edits.
The description of the video features a synopsis, describing the events that take place in the video:
While experimenting with the Tesseract, Bruce Wayne accidentally causes a chain of events which fuses the Marvel & DC Earths. Now combined, the worlds learn to live together peacefully. Or so they think. The future. A dark, desolate world. With such a growing number of super-powered people existing, the Governments of the world decide to initiate the Vigilante Registration Act and attempt to round up all mutants & metahumans. The world descends into disarray. Heroes are forced to take sides - each having conflicting beliefs. Some believe powerful individuals need to be put in check, while others feel they should be given the freedom they deserve. Professor X seeks out Barry Allen, to travel backward in time to stop the Vigilante Registration Act being passed and prevent the events that lead to the abominable future. However, unbeknown to them, a deeper threat lurks in the shadows.