Those hoping for quick answers to the mysteries posed by DC Universe Rebirth #1 in this issue will be sorely disappointed. Batman #21 ultimately does little but give us more mysteries to ponder. It must be said, however, that those mysteries are well-presented and that Tom King's script - based on a plot by Geoff Johns - does a fine job of setting up the story to come.
Apart from the titular Button, there is little obvious connection to Watchmen in the main plot of this issue. Subtextually, there is a mirror story, seemingly unrelated to the main action, which parallels the battle between Batman and the apparent villain of this issue in a way similar to the stories within stories of Watchmen.
The influence of Watchmen is more obvious in the artwork. Most of the pages - saving a few splashes - display the three-by-three grids utilized throughout Watchmen. Jason Fabok masterfully constructs surprisingly detail-driven pictures within the confinement offered to him, with Brad Anderson utilizing a perfect palette to finish it all.
Final Analysis: 8 out of 10. A solid start.