Cat, Rat and Dog

Then Lupin spoke, in an odd voice, a voice that shook with some suppressed emotion. 'Where is he, Sirius?'
Harry looked quickly at Lupin. He didn't understand what Lupin meant. Who was Lupin talking about? He turned to look at Black again.
Black's face was quite expressionless. For a few seconds, he didn't move at all. Then, very slowly, he raised his empty hand, and pointed straight at Ron. Mystified, Harry glanced around at Ron, who looked bewildered.
'But then...' Lupin muttered, staring at Black so intently it seemed he was trying to read his mind, '... why hasn't he shown himself before now? Unless -' Lupin's eyes suddenly widened, as though he was seeing something beyond Black, something none of the rest could see, '-unless he was the one... unless you switched... without telling me?'
Very slowly, his sunken gaze never leaving Lupin's face, Black nodded.
.
.
.
'I trusted you,' Harry shouted at Lupin, his voice wavering out of control, 'and all the time you've been his friend!'
'You're wrong,' said Lupin. 'I haven't been Sirius' friend for twelve years, but I am now... let me explain...'
.
.
.
'I watched you cross the grounds and enter Hagrid's hut. Twenty minutes later, you left Hagrid, and set off back towards 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?'
'And then I saw another dot, moving fast towards you, labelled 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.
'No, Ron,' said Lupin. 'Two of you.'

CAT, RAT AND DOG