In the year 1999, a meteorite struck in the Shibuya ward of Tokyo, Japan, bringing with it the Worm, an alien race capable of mimicking humans - and taking their places. The organization called ZECT opposes the Worm, locating and identifying them when possible, but their ZECTroopers can only handle the larval Worm.
To oppose the Worm's stronger forms, ZECT has developed the Masked Rided System, which gives select individuals the endurance, power, and most importantly the speed necessary to fight the Worm one-on-one. But the Kabuto Zecter chose someone over whom ZECT has no control: a man named Tendou Souji, who fights the Worm for his own reasons and rejected ZECT openly when they tried to recruit him.
There are other Zecters and other Riders, but Kabuto is ZECT's crown jewel and they don't want to leave it in rogue hands. The Worm threat is more important, though, and ZECT's leadership knows it; as long as Kabuto doesn't stand in their way, the Worm take priority. They can deal with Kabuto when it's convenient…
