Picture a catholic school (run by nuns, complete with chapel) in Japan. One of the students (Seira) is a nun-in-training: after school, she tends to the chapel, where people with problems come to pray. She comforts them and they tell her their problems. Seira tells this to her friend, Meimi, who, at night, turns into the Mysterious Thief Saint Tail, to steal from those who got it by hook or by crook, and returning it to their rightful owners. Of course, the police are always after her (she is a thief, after all), and Meimi's classmate Asuka Jr. gets a special permit from the mayor to catch St. Tail.
So begins a sheer endless series of capers. St. Tail is pretty episodic: after the initial set-up, it pretty much turns into the caper-of-the-week show. The villains are always real villains (swindling people out of their property), the treasures range from a girl's teddy bear to the customised running shoes of a marathon runner.
As the series progresses, there's some romantic tension between Meimi and Asuka Jr., and between him an St. Tail (but he doesn't know it's the same girl he teases at school). At one time, St. Tail promises to let Asuka Jr. know in advance where her next caper will be, and every episode from then on, Asuka Jr. gets his warning note in some amusing manner.
There's not really much else to say about this, except that the ending is pretty good: they switch from an episodic to a multi-episode story arc, which ends appropriately dramatic. I enjoyed the ending, the rest is a so-so magical girl anime. A 6.