Don't let the boobs on the cover turn you off – underneath the fanservice element (which will be an added appeal for some readers) is an interesting story about mysterious doors and dark otherworlds. While I wouldn't call this a tame book (although it is for mangaka Hideaki Yoshikawa), it also tries to balance its more exploitative elements with an interesting story about siblings and feeling alone, making this slightly uneven but also quite intriguing.

The story follows high school student Yuuto, who lives with only his kitten Alison for company. His parents are gone, and until seven years ago he lived with his older sister Manami. One day, however, Manami showed him a strange door in a room he'd never seen, and before disappearing through it, she told him that she was a “Treasure Hunter” and gave him a necklace. She forbade him to follow her through the door and then vanished, never to be seen again. Now Yuuto puts on a show of being a normal high school boy, but in reality he has been searching for Manami ever since, Alison his only real companion. This sets Yuuto up not as a bland harem lead, but instead as a character driven by loneliness. His barely-there interactions with his friends are balanced by the way he talks to Alison, seeing himself and the kitten as basically the same, both having been abandoned and left to fend for themselves. He carries Alison with him everywhere, trying to find the door that Manami left through nearly every moment he's not in school.

Of course, one day he does find it: it reappears in front of him while he's standing at a lookout point in town. Two people come through, though neither are Manami – it's a different pair of siblings, sisters Saaya and Hina. Given that the next two characters we meet are Treasure Hunter brothers, this seems to indicate a blood link to the door and the profession, but also speaks to an underlying theme about sibling relationships and looking out for one another. Although Hina and Saaya want nothing to do with Yuuto at first, Hina is a cat fanatic, and quickly falls for Alison. Through her, Yuuto is able to track the girls' next opening of the door, and he follows them through. The tragic results on the other side not only set up the story itself, this volume being very much an orientation to the basic plot, but also establishes the parallels between Saaya and Hina and Yuuto and Manami. The fact that it is purely platonic is helpful to the emotional undercurrent of the story, as it allows for the potential of a romantic subplot while also keeping things action-based rather than distracting us with a forbidden love angle.

At this point Hina and Alison far outweigh Yuuto and Saaya in terms of likability and character development, although that could be deliberate in order to give readers a stake in the unfolding events. Saaya is the most difficult character to take at this point, and while she doesn't fall cleanly into any of the “—dere” categories, she does have elements of them in her quick temper and lack of forethought to 90% of her actions. This does give Yoshikawa the excuse to get as many hands and eyestalk/tentacles on her enormous breasts as possible, and several of the scenes of her being squeezed and groped feel unnecessary in terms of story, as if Yoshikawa felt the need to stop and give us a show before moving the plot along. If that's something that appeals to you, he does it decently well, although there are definitely a few places where we ought to see nipple, given the position of her shirt. I mention it because it seems odd that Yoshikawa would shy away from it, given the fact that Saaya apparently doesn't wear underwear.

In terms of the art, it is serviceable without being particularly good. The monsters that dwell beyond the door are actually better and more creatively drawn than the people, although it is easy to see that an effort has been made to make everyone's outfits interesting. It is amusing that Saaya and Hina refer to their clothes as “protective suits” when they barely cover anything (Hina appears to have forgotten her pants), but that may be a translation/adaptation issue, since later dialogue indicates that it is the fabric, not the amount of covered skin, that is important. Most annoying is that I couldn't figure out if Saaya's bottoms were a skirt, shorts, or a skort, but the bigger issue is that there isn't a great sense of movement, with a lot of running or fighting scenes looking static. On the plus side, the art does not fall into the trap of making the girls only distinguishable by breast size, and the straps on Hina's cap are a neat way to evoke cat ears without giving in to standard fetish headbands.

The Other Side of Secret's first volume is probably not the best to judge the overall story by, since it definitely feels like it is simply setting up the overarching story, but it does introduce us to an intriguing world and establishes a theme of siblings' platonic love that will be interesting to watch develop. At this point it looks like it will combine fanservice with plot, making The Other Side of Secret worth keeping an eye on.