"You've done it before." The Master looked at Romana, his expression dark. "Dire times, yes, yes, but still. Why shouldn't I expect that you'll do it again?"
He took a long moment to glare at Romana before he sulkily followed the Doctor, still not convinced they wouldn't just lock him in and conveniently forget about him unless they needed to provide food or water or tea. But with a weapon aimed at him, his other choice was apparently to die quickly and painfully, and he'd rather have the chance to talk the TARDIS into letting him out, even if his captors didn't want to.