"Batman: Battle for the Cowl: The Underground" begins with the words, "This story is a mystery, the best kind," which is both true and false. This story is a mystery, that much cannot be denied, but it is not the best kind as the mystery in question is "What purpose does this comic serve? Why does is exist? Who would want to read this pointless dreck?" Okay, that's three mysteries, but they're all tied closely together, so we'll treat them as one -- a mystery I'm determined to solve right here and now.
On the surface, this comic's purpose is to fill readers in on what the Riddler, Penguin, and Two-Face are all doing during "Battle for the Cowl." While this happens in a technical sense, their actions create new questions since none of them are really doing anything of consequence. The Penguin hires the Riddler to find the new Black Mask, while Two-Face plots with himself to kill everyone and take over as Gotham's crime lord. That seems like enough to fill a comic worth purchasing, no?
If we dig a little deeper, "The Underground" is actually about how the entire criminal element act in "Battle for the Cowl" with Catwoman siding with the (ugh) "Network," Harley Quinn acting as the Riddler's gal Friday, and the escapees of Arkham doing the Black Mask's bidding. Some of these elements could lead to interesting and fun possibilities, but there's so many characters to touch on that none really have time to deliver. You could fill a comic with interplay between Harley Quinn and the Riddler but, here, the pair are given a few pages.
And nothing is accomplished! Nothing really happens in this comic! It's all in-between-the-panels-of-another-comic and for-more-read-these-other-two-comics, but doesn't function well as a whole at all. This is a textbook example of the worst sort of tie-in issue that does little more than bilk readers out of money with a promise of entertainment and something of substance, but delivering only half-formed ideas and promises of better content at some point in the unspecified future.
Normally, this is where I write something like "Pablo Raimondi's art is the best thing about this comic and the only reason to buy it," but it's not worth it. Raimondi's art is well done and skillful. I love his Penguin, a man who exudes an old world air of sophistication hampered by an obvious viciousness and cruelty. However, Raimondi's art, however lovely, can't salvage this comic.
The only reason to buy this comic is if you're already buying every issue relating to "Battle for the Cowl" and, in that case, this one won't disappoint, because there's that big "Battle for the Cowl" logo on the cover and a pointless plot that fits in perfectly with the main book. Mystery solved.