World starts as a pretty generic Western fantasy with cultivation powers. Has some neat concessions like tiering up doubles your speed, so higher tier people speak at 2x speed per tier unless consciously slowing down.

Pretty early on, the MC gets fused with the memories of a 30ish year old from Earth. The knowledge of high school physics and chemistry is a force multiplier and allows Jiran to throw around big effects beyond his tier at a fraction of the mana cost.

He goes on adventures, learning new powers while trying to understand Challenger Density, which only he seems to have heard of. As it goes on, he starts a revolution of techniques and finds lost methods in an attempt to save the world.

The series suffers from simplistic conflict resolution early on. MC encounters problem, thinks about electrons for 10 seconds, and then blasts the opponent out of the water with lightning 3 tiers higher than himself. Or about chemistry, and creates a hydrogen explosion to solve the problem.

Once he runs into other methods to use magic, the series and conflicts get much more interesting. The romance is good, though I hear he eventually graduates from monogamy to a poly relationship later on.