Absolute nonsense. This is why films directed by mental incompetents about time travel should never be made. All the garbage that was thrown into the blender to piece together this narrative doesn't add up to make even 5 seconds of sense. I would normally say spoiler alert here, but really there's nothing to spoil ...
There are so many reasons that time travel films shouldn't exist in the first place, but just asking some basic questions demonstrates that there's no sense to it.
Who was the original Mannix who was born who went back in time? If he begat himself as his own great grandchild, who gave birth to to the original Mannix before he ever traveled back in time? What a crock of junk.
If one person in the world travels back in time from, let's say from the world of 2051 to 1889, that single individual winds up in 1889, but the rest of the world of 2051 that the person has left behind continues without him. In that world, if a bomb has already gone off in 2023, it's already happened at that point in time and all the way into the world of 2051 as an historical fact ... this, unless the time travel of a single individual establishes a separate parallel universe, the entire plot is entirely impossible. Additionally, the person from 2051 remembers the bomb having gone off so even if that person could go back in time to change that and is successful, what happens to the memory they have of having experienced the bomb going off in 2023?
Come on. So many holes.