From Tokyopop:

To survive a fatal train wreck, Sekai Maihime is thrust back in time where she discovers she has hidden powers that are connected to a doll, 'S.' She is both aided and limited by 'S's' vague (but kind) guardian, Sovieul, who is the cause for Sekai's presence in 17th-century France. Sovieul vows to protect Sekai, but offers little information as to why these strange events are happening.

Unfortunately, they encounter Machiavello (also referred to as the Devil) who looks a lot like Sekai's childhood friend and love, Midou. Machiavello covets the powers shared by Sekai and 'S' so he may obtain the power of the Devil R. Sekai only wants to find Midou and return to her time, but she's constantly hindered by her conflicting desires with 'S' and by Machiavello's attempts to obtain her body.

To find Midou, Sekai must cooperate with 'S' and Sovieul to traverse through time while avoiding Machiavello.