Anthea Gray arrived at Hogwarts in the middle of the Marauders' seventh year with eyes of two different colors, the uncanny ability of knowing where every one of her classes is and the professors' names and Trouble as her shadow. With Dumbledore pulling her out of lessons for private reasons, she begins to spend more time out of class than in it.
That's what the Marauders want to find out: why? But they learn the hard way that some things are better left untouched.
(For those of you who read TDTA, this explains who Anthea Gray is. ;))