"You know how to work it?" Harry said suspiciously. 

"Of course I know how to work it," said Lupin, waving his hand impatiently. "I helped write it. I'm Moony -- that was my friends' nickname for me at school." 

Amelia chuckled. "Cleaver."

"You wrote --?"

 "The important thing is, I was watching it carefully this evening, because I had an idea that you, Ron, Hermione, and Amelia might try and sneak out of the castle to visit Hagrid before his hippogriff was executed. And I was right, wasn't I"

"I get the feeling you're right a lot," out of the corner of her eye Amelia could see Sirius give a curt nod.

 Lupin had started to pace up and down, looking at them. Little patches of dust rose at his feet.

 "You might have been wearing your father's old cloak, Harry--"

 "How d'you know about the cloak?"

 "The number of times I saw James disappearing under it...," said Lupin, waving an impatient hand again. "The point is, even if you're wearing an Invisibility Cloak, you still show up on the Marauder's Map. I watched you cross the grounds and enter Hagrid's hut. Twenty minutes later, you left Hagrid, and set off back toward the castle. But you were now accompanied by somebody else." 

"What?" said Harry. "No, we weren't!"

 I couldn't believe my eyes," said Lupin, still pacing, and ignoring Harry's interruption. "I thought the map must be malfunctioning. How could he be with you?" "No one was with us!" said Harry. 

"And then I saw another dot, moving fast toward you, labeled Sirius Black.... I saw him collide with you; I watched as he pulled two of you into the Whomping Willow --"

 "One of us!" Ron said angrily.