It was the third scream belted out by The Joker that snapped Batman out of his trance. He took off the ridiculous mask that had been placed on his face and crushed it in his hand. He then turned to the door that Mad Hatter had carried his 'Alice' into.
It had been the third scream of Joker's in particular to snap him out of Hatter's control because the scream had been cut short, was proceeded by a muffled 'Stop resisting!', and was followed by silence from Joker.
Bruce hurried over to the door and tried to open it - locked, of course. He took a step back before slamming into it. No good. He tried again. "Joker!"
"Oh! Perhaps I gave you less credit than you deserve, Alice." Hatter said from inside the room. Batman expected to hear Joker make some sort of aggressive counter, but the clown prince of crime made no sound.
"Dammit!" Batman charged into the door once more, twice more, and then suddenly the door was opened before he smashed into it, causing him to stumble.
"Have you come to watch, Batman?" Hatter asked. Joker lay unconscious on the floor in front of him, and he had a hand on the clown's pants as if he were about to tug them off.
"Let Joker go, Jervis." Batman demanded.
"Or else what? Face it, Batman; you should be glad I'm taking Alice off of your hands. Alice is mine now. I know Alice will cooperate with me more once I give him one of my lovely hats. I've been saving the best one of all for my dearest Alice... It'll make him mine, forever."
"I said, let Joker go. Don't make me hurt you."
"Oh, but Batman - you won't even be able to touch me!" With a gesture, Hatter pointed out that all of his goons were now entering the room, ready to fight. Batman started fighting them solely out of necessity.
While Batman fought of his goons, Mad Hatter picked up the sickly Joker and made a break for it, which made Batman fight a little more brutally.
Once all of Hatter's goons were unconscious, Batman contacted Alfred. "Alfred, The Mad Hatter has kidnapped The Joker. He's determined to make him his protégé, and he might even kill him if he can't."
There was a brief pause of Alfred's side. "... Frankly, I do not see much of an issue in this, Master Wayne. Though I assume you will try to locate them?"
"Yes. I can't let Mad Hatter kill Joker, because then I'll have to deal with Harley - she'll be much too vengeful."
"Well, be careful, Master Wayne. Good luck."
"Thank you, Alfred." Batman stopped the call and stood where he was for a long moment, thinking about this. He was about to go off on a rescue mission to save The Joker, who had caused him so much anguish and even poisoned him, from The Mad Hatter. Was Joker really worth it?
Bruce shook his head. "It's not a matter of choice." He muttered to himself. "I cannot allow Jervis to kill another 'Alice'."
With that, he ran out of Hatter's hideout and began his search for the psychopath and clown.