When 15 year old Thunder Demon, Ryuka Takamoto, bitterly parts ways with her best friend, 20 year old fire demon Hiei Jaganshi, after years of friendship, her entire life turns upside down.
She gets by alone in the Makai for fifteen years before being spotted by Spirit World police in a clumsy raid for supplies. It's a game of cat and mouse for five years when Koenma, Prince of the Spirit World, sends his team of two Spirit Detectives after her; Shinobu Sensui and Annote Itsuki.
Ryuka evades their traps for five years, before finally falling victim to Shinobu on the night of her 17th birthday. Koenma gives her an offer; the death penalty, or join the two young men and work for him as a Spirit Detective. Her exquisite skills with her sword and unique electric based abilities could be of great use to the team.
Three years go by, and the three young Spirit Detectives are close knit and inseparable. Shinobu and Ryuka kindle a hidden romance, until one night when a mission goes terribly wrong... Shinobu begins to lose his mind, becoming increasingly violent and mentally unstable, developing strange personalities...
Ultimately, Shinobu is forced to retire on Koenma's strict orders. Itsuki resigns as a result, refusing to work for the Spirit World without Shinobu, his best friend. The two disappear seemingly into nothing, without a trace.
Despite his desperate attempts to persuade Ryuka to take Shinobu's place as head Spirit Detective, she declines, heartbroken over the loss of yet another soul she was close to. She does agree, however, to continue working under cover as a Japanese High School student, keeping her eyes and ears connected with the Spirit Word as a sort of secret agent.
Ryuka does this for seven years, repeating the 12th grade at seven different High Schools, acting as a sort of radar for demon activity, which was usually low and easily exterminated... That is, until word spreads through the Spirit World community that the Chapter Black tape has been stolen, with Koenma's suspicions pointing towards none other than Shinobu Sensui.
Along with the missing tape come a strange new species of bugs, untraceable to the human eye, but fully visible to demons or immortals. An obvious sign that something isn't quite right...
Ryuka's suspicions are confirmed when Botan, her best friend, surprises her in her favorite coffee shop one evening after school, an "emergency" on her hands, requiring Ryuka to accompany her to an unknown location at once. Just like always, Botan provides her with little information, only that it's imperative she come with her. So, of course after being given no choice, Ryuka abides by her friend's wishes.
When she arrives at Sarayashiki High School, she meets Kuwabara, a member of the new Spirit Detective team, and it isn't until then that she realizes the trouble that the world is in... And that Koenma assigned Ryuka to work with the Spirit Detectives, solving this strange, dangerous mystery... There's just one problem they have to solve first, and that's to free Yusuke Urameshi, the lead detective, from a group of kidnappers, believed the be a group of powerful human teens...
After a close call inside the mansion of four dimensions saving Yusuke's life, Ryuka finds herself glued into the position of member #5 on Team Urameshi, completely against her will. But, with no other option and the fate of the world at the hands of a sinister group known as "the Sensui Seven", there isn't much she can do...
Even though, as if things aren't already crazy enough, she recognizes a certain fire demon on the Urameshi Team all too well, and isn't particularly happy about it... But time is running out, so they'll have to set their differences aside and work together as a team.
The fate of the world depends on it.
-Continued in depth in: -Chapter One-.