It's actually really good and if you like Danmachi you should see if you can find a version of it and give it a read/listen (it used to be on Youtube but I think they've taken it down). The long and short of it is that Bell accidentally falls through a time-skip, and winds up in the past where he takes on the name "Al" and is taken in by the Astrea Familia.
He weighs up his conflict of changing the past vs helping people, and (obviously) ultimately decides that he can't let people suffer before his eyes if he can prevent it, even if that means his future doesn't happen anymore, and so he throws himself into Orario's Dark Age, helps save the Astrea Familia, helps save Shakti's sister, Ardee, and helps bring the Dark Age to an end.
By the end of it, he's unable to keep himself in the past anymore, so returns to his own time, where he's sad to find everything is still the same. But we're given a glimpse now of an alternate past where, even though the memory of "Al" is slowly being erased, the changes he's made aren't, and the people he saved live on.
This obviously has him not just save the Astrea Familia, but breed them. A change I'm pretty okay with. Although, it'll mean the Bell of that time will certainly have a lot to deal with when he comes to the city in 5 years...