Nine years before the tragedy that made them deadly enemies in X, Subaru Sumeragi and his friend Seishirou Sakurazuka are psychic investigators in Tokyo. Together with Subaru's sister Hokuto, they must fight mystical forces and serial murderers.
Tokyo Babylon was an early CLAMP work. I'm not a great fan of one-off OVAs, which rarely have enough meat on them to stick in the memory, but these are okay. It helps to have watched X, not just for the poignancy of knowing the characters' fates but also because otherwise their "out of the blue" magical powers might have felt like a bit of a cheat.
There's less "CLAMP prettyboy" in the air, which is good but means that our heroes don't always resemble the X versions of themselves! The voice actors are different too. Hotoko is cute and the stories do enough to keep you interested despite being throwaway one-offs, but on the downside the soundtrack has a song in appalling Engrish. To be honest Tokyo Babylon isn't anything special. Nice animation and nice stories, but nothing that grabs you by the throat. However if you're about to watch X, it's probably a must-see. It has no plot relevance whatsoever, but you'll get a deeper insight into three of its characters. It's... well, it's nice.