Lethal Operation Rei is a sequel game to Lethal Application and a sister game to Lethal Operation Rugitta. The game was released at Comiket 89 featuring overhauls to the game engine used for Lethal Operation Rugitta such as bug fixes and smaller mechanical changes which would eventually be patched over to the sister game.
Very much like it's sister game, Lethal Application Rei plays very similar to Lethal Application Rugitta with the exception that Rei is now the main protagonist as oppose to Rugitta. Rei herself is a lot more offensive oriented compared to Rugitta who was generally more balanced out in comparison.
A lot of the mechanics, stats and characters are still maintained within the game such as being able to equip different shots to most of the characters in the game and the Sphere Tyrunt system that was also present in Rugitta.
However there are a lot more stages unique to Rei which are not found in Rugitta and vice versa with some stages from Rugitta being absent in Rei mainly due to where each character goes in their own story.
The game takes place 10 years after the Ground-Sky War and the world has split up into four different factions being Northside, Southside, Eastside and Westside. The story starts off with Rei being woken up by Rugitta who found her unconscious in a field. Rugitta attempted to fix up Rei to which Rei asks if she is able to handle to which Rugitta replies she came here to see if she can do it without much to really back it up. Suddenly someone in pursuit of Rugitta managed to catch up with her to which Rugitta then decides to leave Rei in the hands of someone else, that someone being Doctor Doal.
After Doctor Doal comes to repair as much of Rei as possible she sends Rei out and back from Westside to see if she can be rehabilitated. After coming back from Westside and after Doal tries to call Rugitta back the lab then goes under attack by forces from Southside claiming that Doal repaired a Skyside weapon, that being Rei.
Rei decides to drive off the invading forces much to Doctor Doal's dismay which she's eventually assisted by Rugitta in driving off the invading army. Afterwards Doal wants to take them both in for maintenance though Rugitta is already set on trying to save the world and also to check on the Oju trees.
After getting maintenance from Doal after the fight, she was surprised by the amount of damage Rei sustained despite Rei herself still being fine. Though Doal cannot do much more for Rei since she's unfamiliar with how war machines work but she tuned Rei up well enough to sustain her for the long trip Rei is setting out on.