The new world is a lie.
Every isekai story out there is nothing but the fabrication of a dying soul who has lived an unfulfilled life and died a traumatic death. It is the final self-preservation failsafe of a weakened soul in reality so that they may endure long enough to return to reincarnation cycle.
Normally, these would resolve themselves one way or another so nearly all of these deaths have no effect on the real world but when a dream goes on for too long and their spirit somehow remains bounded to the earthly plane, things from the dream can start leaking out of it and become part of reality.
This is where specialists come in. There is an ancient order dedicated to curbing the threats this may cause by invading these dreams and forcing the soul to a conclusion. Without them, the world would be a much more fantastical place that'd only breed more and more of these corrupting influences on reality. And unfortunately for these specialists, these dreams are more often than not fatal for the would be saviours of the world and veterans come out of them changed by the lifetimes they live out inside and the choices they must steel themselves for the sake of the mission.
The mission that take center stage concerns a very curious case that require the hands of hardened veterans. A couple had committed lover's suicide and both their dreams have persisted for far too long and expanded so much that they are beginning to merge together. Cross contamination has been documented before but never at this level and it brings up worrying possibility - what if they go on to merge with other dreams?