It's an Elseworlds where Bruce Wayne is Green Lantern. He's brooding in his study and about to say, "YES, FATHER, I WILL BECOME A BAT!!!!" when suddenly the dying Abin Sur appears and makes him the next green Lantern of Sector 2814. In other words, DC's most famous non-superpowered hero has been given a magic ring that can put its wielder up towards Superman's power levels. It's been called the most powerful weapon in the universe, which isn't an absurd statement. It's powered by willpower and it lets you create glowing green constructs of anything you can image. It also lets you fly.
Why tell this story? What's Barr exploring with this? Answer: not much and it's not very good.
In synopsis, it's fine. Unimaginative, but fine. The pace is fast and the events are adequate. People get killed, absorbed, etc. You could use it as a skeleton to adapt into a reasonably good TV episode of some cartoon or other. Green Lantern's arch-nemesis Sinestro kills some of Batman's friends for underwritten shock value, finds Joe Chill, creates a version of Two-Face and ends up becoming an unconvincing token attempt at the Joker. (He wears a purple suit and he's an arch-enemy who escapes at the end. That's it, really.)
As for Batman, he doesn't follow orders. Those little blue dwarves on Oa send four other Green Lanterns to take him down.
The problem is that Barr has a tin ear for dialogue and the whole thing feels like a first draft outline that accidentally got sent to the artist instead of the real thing. This version of Jim Gordon is a dick and his lines feel like writer's shorthand. He'd have become a real person in a later draft.
Surprisingly, this version of Batman has appeared elsewhere. He's a playable character in Lego Batman 3: Beyond Gotham. He's also in the dumb-sounding Countdown: Arena, because these days his Elseworld is in Earth-32 of the 52 universes of the Multiverse.
Nope. Don't care. It's uninteresting and underwritten. J.M. DeMatteis had won a Comics Buyers' Guide Fan Award the previous year with a Batman-Superman amalgamation in Superman: Speeding Bullets, so presumably DC were hoping that lightning would strike twice. It didn't. Even the art's forgettable.