NOTE: This whole story happens before the Batman/Superman teamups, and this is Slade's first try in the Deathstroke costume. This Deathstroke has a resemblance to and acts like the Marvel Comics character Deadpool and makes a few references to him.
The story opens up with Slade being hired to kill Bruce Wayne. He mentions a "costume" that he wants to try out. We later find out that he is referring to his Deathstroke outfit. Meanwhile, Superman and Batman are in Metropolis, battling alternate versions of themselves--and some other Justice Leaguers--that seemed to have arrived there via inter-dimensional rift. They send them back to their dimension, and have a lovely chat about their secret identities. Batman says that he is looking through every college yearbook, and that HE WILL FIND that chin. Superman compliments Batman on the lead-lined cowl. Moments (pages) later we see Clark Kent and Lois Lane, preparing to go on a short cruise for an article about the cruiseship. Later we find out, just by coincidence, that Bruce Wayne is on the same cruise. A computer error puts Clark and Bruce in the same room. At dinnertime that night, we see Slade drop two pills (presumably poisoned) into Bruce Wayne's Martini. However, before he can drink it, the glass explodes from a nanosecond blast of heat vision from across the room (winkwink), so short that nobody can see it. Later that night, we see a very awkward scene involving Clark and Bruce sharing a bed. Then, the night is shattered by a scream. They both start making excuses so they can go out and "suit up" without the other one noticing. Then, Bruce takes the offensive, trying to shoot Clark with a tranquilizer dart. Naturally, it bounces off, and something clicks in Wayne's head--the chin. At this point, Deadpool jumps in (but DC won't admit it is him because Deadpool is a Marvel character). He jumps in front of a window, behind which is Slade, with a gun. He shoots Deadpool. We go up to the deck of the ship, where Lois stands as Ultraman and Owlman (the interdimensional dopplegangers that we met earlier) make their grand entrance. Superman jumps into his costume and rushes up to the deck to save everyone (like usual). We see Wayne running, presumably to the batboat, when he is attacked by Superwoman (alternate Lois/Wonder Woman, who we find out was also paid to kill Wayne. He is, once again, saved by Deadpool. The fight continues, and we see Wayne dive into the water (to the boat). This is where the little black capton boxes describe an extraordinary fight(the caption boxes seem to be those of Deadpool), involving Darkseid, Thanagarians, and many other heroes (and villains), all with their inter-dimensional lookalikes. After this, Superman races and saves the day (again) in a very complicated fashion. When Lois confronts Superman and Batman about their secret identities, the jig is up. Well, not quite. Well Lois is talking she looks over and sees Clark and Bruce (wearing nothing but boxers and a bathrobe). She storms off. We later find out that Clark flew himself and Bruce at mach 4, changed their outfits, and brought them back, all in a split second. But who was behind the inter-dimensional rift? Well, it was Mr. Mxyzptlk, of course! Our story ends with that pesky pixie from the fifth dimension speaking with a Bizarro producer.
Info on this issue provided by Justice Warrior and Deadpool.