Key to Dimension

I picked up a key. It should be... Of no use. Hey! Here are... Pitch black bullets! What the hell? The key to crossing the dimensional barrier? But... Forget about time travel, as an otaku, although I am surprised, it is not unacceptable, but why did I become a child? Well, time travel happened, and it's not a big deal to go back to your childhood. But... The problem is, we are a young lady, not a lolita... How can we fix it! This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.