Not too much is known about the plot of James Gunn's The Suicide Squad, with only hints at the potential secret villain of the film and the misfit misanthropists killing their way through a jungle island having been released. It was later confirmed that the island Task Force X lands on is actually one familiar to fans, Corto Maltese, which first appeared in Frank Millar's The Dark Knight Returns.
Additionally, while on this island, the team - consisting of Margot Robbie's Harley Quinn, Idris Elba's Bloodsport, John Cena's Peacemaker, and many more - will be tasked with destroying a “Nazi-era prison” called Jotunheim. In the comics, this was a fortress introduced in the first issue of John Ostrander's Suicide Squad, which Gunn has claimed was a huge inspiration for the film.
A new synopsis gives more details about the plot of the film and how the Squad lands on Corto Maltese.
THE SUICIDE SQUAD DROPPED IN ENEMY TERRITORY
An official synopsis for James Gunn's The Suicide Squad has been released, reaffirming that the movie will take place on the island of Corto Maltese and teasing characters like Margot Robbie's "psycho" Harley Quinn:
“Welcome to hell — a.k.a. Belle Reve, the prison with the highest mortality rate in the US of A. Where the worst Super-Villains are kept and where they will do anything to get out—even join the super-secret, super-shady Task Force X. Today’s do-or-die assignment? Assemble a collection of cons, including Bloodsport, Peacemaker, Captain Boomerang, Ratcatcher 2, Savant, King Shark, Blackguard, Javelin and everyone’s favorite psycho, Harley Quinn. Then arm them heavily and drop them (literally) on the remote, enemy-infused island of Corto Maltese.”
The synopsis further details that Colonel Rick Flag will be the only government official on the ground keeping tabs on the killers, villains, and mercenaries:
“Trekking through a jungle teeming with militant adversaries and guerrilla forces at every turn, the Squad is on a search-and- destroy mission with only Colonel Rick Flag on the ground to make them behave...and Amanda Waller’s government techies in their ears, tracking their every movement. And as always, one wrong move and they’re dead (whether at the hands of their opponents, a teammate, or Waller herself). If anyone’s laying down bets, the smart money is against them—all of them.”
ONGOING REBELLION IN THE DCEU?
The reason that Corto Maltese is revealing is that it's an island off the coast of South America home to a rebel uprising, based on comic lore. While the United States backed the local government, the Soviet Union backed the rebels. It even got to the point that the United States Government sent in Superman in an attempt to put a stop to the rebel forces before a nuke launched by the Soviets temporarily took him out.
Additionally, something interesting to note is that James Gunn considers The Suicide Squad to be a sequel to Ostrander's comics rather than an adaptation. This could mean that the events of those comics could have already happened in this movie universe. After all, Bloodsport even tried to shoot Superman with a Kryptonite bullet at one point in his mercenary career, so who's to say what else has happened in this universe that audiences haven't seen.