Since Mugen Himitsu is a one-shot my review will be short. The part that stands out the most in this particular series is the beautiful art work. It's crisp and detailed and contains a lot of the positive style traits one expects of shoujo manga. The storyline is fluid and transitions nicely from one scene to another.
As for the plot, I love how the manga opens up like many of my favorite mystery series and the suspense is nice as well. The interactions between the two main characters is also interesting and the plot flows nicely from beginning to end. It isn't just the pacing
of the plot that was good in this particular one-shot but also the unfolding of the romance. The story could have used a bit more fleshing out, but was also quite good for a one-shot.
I also enjoyed the characters. Without going into much details, it was nice to have a feminine character who isn't moping about not having a romantic interest, or even thinking about how she doesn't need a romantic interest because she isn't that kind of girl. Instead, she focuses on what is going on around her and reacts accordingly.
The one-shot was also a nice, quick read.