Jack hesitated a moment. But he wasn't getting any nasty vibes from the TARDIS and only - odd - ones off the Master, so in the end he nodded and stepped inside. Gazed around at a TARDIS that was very similar to the one he knew on the inside as well as the outside, and strode up the ramp to the console.
He turned his back on it, folded his arms, and looked at the Master with a challenge. "Well?"