The basics you need to know to understand what's going on:
Nightwing rejoined the team.
Wally West was resurrected by a pair of dead sorcerers named Stefan and Vladimira Dracov, who sent him to their son in the world of the living, Nikola Dracov. Nikola revealed to Wally that he was an old circus friend of Dick's and that he needed to find him to warn him about something-the two set off to find Nightwing. Along the way, it became clear that Niko not only had actual powers, but that they seemed to be coming from a strange ring he wore. He told Wally that the reason he wanted to warn Dick was that he had seen a vision of the acrobat dying. Unfortunately, they got there a little too late, and Niko didn't get the chance to warn Dick before the latter was killed. Shattered, Niko set about using his strange powers to bring his friend back. Long story short, he succeeds, and ends up joining the team, taking on the name Dark Caster.
A debrief of Niko's powers: The ring is not what gives him his powers-it's how he channels them. His family is supposedly descended from a demon of darkness, and each one of their lineage serves as a current for all the Dark Power in the universe, and in order to keep that power contained, they each wear a magical ring. If he were to remove it, he would lose control of his powers, tearing apart the time-space continuum. Niko also has the ability to speak to the (aptly named) 'Dark Powers,' who serve as a manifestation of what gives him his power. They are not exactly sympathetic, but it is in their interests that he lives. His abilities give him powers similar to telepathy that rivals M'gann's. Also, Niko has a crush on M'gann, but refuses to admit it, especially since (thank you to someone on TV Tropes who made me realise this makes sense) he knows she's subconsciously shaping the minds of people around her.
The Team finds another genetic experiment called Anima, a young woman who has been infused with animal abilities, giving her strange black marks on her skin. Though she is originally ordered to kill them, she relents, making her own decision to join them. The Team name her Angel Libre, using Anima as her alias, and she develops a large crush on Connor. Unfortunately, as he and M'gann are back together, she's unable to approach him, and because of this, develops a close friendship with La'gaan.
During a battle with Darkseid, two women appeared and turned the tides in the heroes' favour. One of them was a former, Hispanic Navy SEAL with a penchant for explosions, the other a former research student who had been genetically infused with nanobots. They both vanished into the crowd after the victory. Later research by Batman revealed them to be going by the names Firework (the Navy SEAL), and Quantum (the scientist).
The Team found out that there was another clone of Roy-except this one was female. Named Raya Harker, she's raised as Lex Luthor's adopted daughter until she discovers the truth, and, with Niko's help, saves the rest of the team from Luthor with a shotgun (Raya had the gun, not Luthor), before running off with the help of the other two Roys because she didn't want to be analysed. She has yet to return, but she made sure the other two would keep in touch with her.
Finally, the reason this story is happening-in an attempt to save everyone's lives, Niko and Dr. Fate transported them temporarily out of time. They end up watching a possible future, like they're watching a movie. And so, the story begins...
One last warning: If you are a die-hard fan of SuperMartian, I tell you now-I break them up. For reasons I can't exactly explain, I cannot stand the pairing. It just doesn't do it for me. Don't worry, I give them both partners who (in my mind), are pretty goddamn awesome. If that's the only thing preventing you from reading this, then just think of it as a possible future. Also, there is no Chalant in here. Sorry. I sort of see them as bygones-be-bygones.
The sea floor shuddered with a heavy quake, and another followed quickly on the other's heels. The group watched in amazement as Atlantis's bright towers shook, but did not topple. Still, this city was different than before-sleeker, older.
When it wasn't being bombarded by giant submarines.
The hulking machines were currently too far away to topple the city, but the force and power of the shots they were issuing left no doubt that they were entirely capable of it. At the walls of the capital itself, black-clad soldiers were locked in battle with the royal guards.
"Stand your ground, are you soldiers or aren't you?!" Shrieked a female voice as a slim, blue-green figure flitted between the enemies, taking them down with the speed and poise of an expert warrior. She paused, hovering in the water above the wall, long silver-white, tentacle-like hair streaming out behind her, red eyes surveying the battle below.
Everyone looked at La'gaan. "Small guess who's kid she is." Wally commented. Indeed, the resemblance she bore to Lagoon Boy was too clear to ignore.
As they watched, the girl clenched her fists, narrowing her eyes. She enlarged in a fashion similar to the way her father did, crashing down atop a garrison of enemy soldiers, throwing several more out of the way when they charged her. One of them, however, got a blaster behind her, firing just as she turned. The explosion shook the wall, sending hairline cracks through it, and the soldier lowered the blaster.
That was a mistake.
A stream of water flashed out and wrapped around his arm, yanking him sideways and slamming him into the way, hard. He slumped to the ground as the sand cleared, revealing not only La'gaan's daughter, but another girl as well. Tall, strongly built, with dark skin and her hair floating behind her in a thick, blonde braid. Swirling tattoos covered her arms, and she carried a familiar pair of water-bearers.
"Tula!" The first girl cried happily.
"Head in the game, Mera." Tula replied, calling the water back to her. "Those submarines are getting closer." The two looked around at the battle, and something was painfully clear: Atlantis was losing.
Mera frowned. "The outer wall isn't going to last much longer." She told Tula. "We should evacuate the sectors immediately adjacent to it." The other girl, who had to be Kaldur's daughter, nodded.
"Do it. I'll explain to Garth." Mera turned, streaking down to a group of soldiers who were relatively inactive, taking care of their attackers easily.
"You! Begin evacuation of the outer sectors, and do it quickly!" One of the men's mouths twisted.
"Why should I do what you-" With a cry of outrage, Mera grabbed the man by the collar, shaking him and shoving him into the wall.
"Because I am the Acting Lieutenant Commander, as appointed by the Lieutenant Commander, who just so happens to be my father who was appointed by the Commander, whose son, the Acting Commander, requested my installment! If I tell you to evacuate the outer sectors, you evacuate the outer sectors!" She released the stunned man, who saluted her quickly.
"Y-yes, ma'am!" He and his comrades swam off while she returned to the fight.
The scene faded, blurring into the palace, which was awash with military officials. Aquaman frowned, folding his arms. "I don't understand. If Atlantis is under attack, where are we?"
No one wanted to try and answer.
"Acting Commander Garth!" One of the men in the hall turned around at the shout. He was similar to appearance to Tula, with slightly shorter hair that reached the edge of his shoulder blades in a ponytail, and pale eyes that stared out of his dark face.
"Report."
"Sir! Acting Lieutenant Mera gave orders to evacuate the outer sectors! She and your sister think that the outermost wall may not hold much longer!" The boy's mouth twisted, but he stayed calm.
"Of course. Obey them as you would me." The soldier nodded and raced off while Garth turned back to the assembled Generals. "It would seem we have another predicament. If they can breach the outer wall, what are the odds that the other walls will stop them?" He looked around. "Gentlemen... I fear we are fighting a losing battle." The doors opened again. With a sigh, Garth turned to face the new arrival, and they were all shocked to see the elderly court advisor.
"My lord Garth," The old woman wheezed urgently, "I have terrible news."
"Ma'am, the outer sector is clear!"
"Good!" Yelled Mera, turning to Tula. "We need to fall back! The wall's about to come down on top of us!" Tula nodded, whirling to give orders to the troops.
"Fall back to inner wall!" She called, starting an echoing chain of calls of 'Fall back!' and 'Retreat!' Together, the two girls turned and sped away-just as the outer wall came crashing down. To their surprise, they made it back to the second wall mostly intact, regrouping and giving orders. Tula looked at Mera.
"Mera, tell Garth the wall was breached. We need a plan, and we need it an hour ago!" La'gaan's daughter nodded, streaking away again.
"What? No. No... This-this can't be happening! Not now!" Garth was pacing the meeting hall, shaking his head. Finally, he stopped, turning to the advisor with desperation. "What do we do? Without the royal family, who's in charge?" The old woman gazed at him sadly.
"You are, Commander." For a moment, Garth was too stunned to reply.
"M-me...?" He whispered, then snapped out of it. "N-no. No. I'm not Commander. My father-" He stopped when he saw the woman's face. "... Gods, my..."
"I have another piece of news, my lord..."
Mera swam easily towards the palace, bursting into the great hall with very little ceremony and streaking down the corridors.
They found it easy to forgive her rudeness-Atlantis was clearly in dire straits.
"This is horrible..." Murmured M'gann, laying a hand on her ex-boyfriend's shoulder. He shrugged it off, but allowed Angel to pat his arm. M'gann bit her lip uncomfortably, looking up between the glowing forms of Niko and Dr. Fate on either side of them, wondering if they were seeing this, too.
Mera threw open the doors to the hall, and froze, taking the scene in. Then she raced to Garth's side, touching his shoulder. He looked up at her hazily.
"Mera, I-um..." He swallowed, straightening. "Lieutenant, report." She fell back, nodding.
"Yes, sir. We've had to pull back to the inner wall due to excessive casualties. The outer wall has been breached, and we're not certain how long the inner one will last." He nodded back slowly, his blond eyebrows pulling together.
"... How many have we lost in this action...?" Mera hesitated, then whispered a number to him. His face paled beneath it's dark brown tint.
"That bad?" Aquaman commented. "They don't have many options." Kaldur looked at him.
"My king, are you saying they should...?" Aquaman nodded solemnly.
"It may be their only chance at survival."
Garth turned towards the windows, gazing out at the approaching army sadly. Mera gazed at his back sadly. He swallowed again, turning back.
"My friends, we are low on choices. We could continue to fight, to stand in the face of overwhelming odds. I, however, do not believe this for the best." He took a deep breath. "I have recently been informed that by the current circumstances, full command has fallen to me." Mera's eyes widened. "The royal family is gone, and as for the others..." He trailed off, his expression that of someone who had just eaten something bitter. Then he collected himself. "General Coor'ea?" One of the Generals swam forward.
"Yes, sir?"
"... Give the order to retreat to the Crystal Caverns." Shocked silence filled the room for a full minute.
"But... But sir!" Spluttered the General. "To-to retreat would be-" Garth, pale blue eyes fierce, rounded on him.
"Cowardly? Shameful?" He hissed. "Look out that window, General. Our people are dying as we speak! How much longer do you think they'll last against that? If we try to stand and fight here, we die. Our people die. Atlantis will be massacred, devastated, and destroyed. There will be nothing left. Not one man, child, or woman. All will be dead. If we retreat now, we live. We wait. We bide our time, grow in numbers and strength, and then strike them when they're at their weakest. We form allegiances, watch, and learn. But most importantly, we live. Our people live. I make this choice for them-for those who fight out there, waiting for our decision. For those who rely on us. Therefore, General; give. The. Order. To. Retreat." By the end of his tirade, his voice was a dangerous whisper. The older Atlantean nodded meekly, fleeing without another word. Mera, however, hesitated.
"B... But Garth... The Deep Zero Command Unit was sent to the Grotto. If we open the Cavern gates, that entire rock-face will be crushed." For the first time, her voice sounded feeble and emotional. Garth looked at her briefly, then turned away, taking another deep breath.
Beast Boy frowned. "I don't get it. What's the 'Deep Zero Command Unit'?"
"It's an elite military unit that answers only to the King." Kaldur explained. "They're used for only very important missions." They looked back at the scene before them. Slowly, grim understanding showed on Aquaman's face.
"The other piece of news..."
"... They're dead, Mera. Let's keep the living safe." He, too, swam away. Mera hung in the water for a moment, her eyes moving to a giant cliff that stretched out to the west, her face frozen in broken horror.
"... N-no..." She breathed, her voice cracking. Then, suddenly, conviction poured over it. She steeled herself, turned, and sped away through the water, even faster than Garth had.
People streamed out of the city quickly, herded gently along by soldiers and guards. Despite the large number fleeing, the streets were littered with corpses, as was the sea floor. Mera flashed over to were a group of men were attempting to free a lever from the rock. "Move!" She shrieked, the markings on her upper arms glowing as she grew. They dove out of the way as she seized the control, pulling it easily. The stone began to move, sliding and crushing, a large, gaping black hole opening in it. The citizens swam through, and Mera was quickly joined by the brother and sister.
"The city is clear." Tula told the other two. "And they do not appear to be pursuing."
"They cannot see where we fled to." Garth replied. As the other people finished pouring into the Caverns, the three stayed behind, watching as the monstrous submarines slammed into the city. There was a moment of tired, sorrowful silence before any of them spoke. "Mera, Tula." Garth said, taking another deep breath. "Go contact our allies on the surface. Tell them Atlantis... Has fallen."