The seventh episode in the anime series Big O. The previous episode is A Legacy of Amadeus.
At the edge of Paradigm City, there is a harbor, where fish are caught. At the bottom of the harbor, there are a multitude of ruined buildings and skyscrapers-- another mysterious legacy of the Event from 40 years prior. In one of these buildings, air is trapped, and electric light still illuminates. And in one of the rooms, Roger Smith and Angel are trapped, wondering if they'll manage to escape, and wondering how long it will take for the air to run out.
A day prior to this sad scene, Roger had gone down to the harbor with the intent of finding out why the price of fish was skyrocketing, and to negotiate a price break with the fishermen if possible. A local shipbuilder tells him that the prices are being inflated by the shops, due to the diminishing supply. All of the boats are in good working order, and the harbor is still well-populated with fish-- but the fishermen won't fish, because they fear the wrath of the sea titan's minions.
In the room at the bottom of the ocean, Roger and Angel spar. She pulls off her suit due to the heat, and he wonders if it was even a good idea to bring her along.
At the docks, Roger doesn't buy the story of the sea titan's minions menacing the locals. He bribes the shipbuilder to make him a boat that can travel underwater-- but the boat is already built! It seems that the shipbuilder somehow remembered how to create the two-person submersible, claiming that it might have been a legacy of his father. It's not quite in working order, so the shipbuilder tells Roger to come back the next day. As he's leaving, Roger spies the "minions of the sea titan"-- they're fairly obviously human divers, in somewhat fearsome-looking wetsuits. He gives chase, and they immediately flee.
At the bottom of the ocean, Roger notices that Angel has a pair of odd scars on her back. He asks her if she actually comes from Paradigm City; she seems annoyed at his question. Angel claims that she's only working with Paradigm to unearth forgotten memories and artifacts for herself, because she desperately wants to remember the past. She questions the complacency that the residents of Paradigm City have towards the past, and Roger explains that ignoring the past is the best that the people can do to get through the day.
At the docks the next morning, Roger goes to retrieve the submersible-- but it's gone! The shipbuilder says he sold it to a higher bidder, who just happens to be leaving the docks-- it's Angel! Roger runs after the submersible, and manages to leap right into it as it pulls away. This annoys Angel, but Roger is adamant that he should be allowed to go with her.
The two head down in the submersible, and marvel at the ruined city at the bottom of the harbor. The lights in many of the buildings eerily still work, and Angel wonders if people still actually live at the harbor's floor. Her question is answered quickly-- the submersible is shadowed by the same divers from the night before, who plant a mine on the sub. The damage results in the sub crashing into the lobby of a building, bringing Roger and Angel to their current sad state.
The pair watch, fascinated, as the deep-sea divers ride a tow cable down a deep hole from which a bright light is emanating. Angel loudly complains that these divers are going to take all of the memories; Roger says that some memories shouldn't be disturbed. His hunch is right; there's a huge rush of rising water, and a slimy monster emerges from the depths. Roger summons the Big O.
Meanwhile, Paradigm CEO Alex Rosewater is watching. He observes the beast, decides it isn't sufficiently harmful, and orders the military police to hold their fire for the time being.
Meanwhile, Roger is surprised to find Dorothy in the cockpit of the Big O; she explains that her coming along would have made sense, as she could have operated without the benefit of oxygen. Roger rescues Angel, and rushes to meet the sea titan, who surfaces and starts blundering around the city, causing damage. The creature isn't specifically attacking the citizens, just wandering around and moaning mournfully. Dorothy remarks that the creature is simply unhappy, because it has lost its master.
Against the wishes of Rosewater, Dan Dastun orders his men to open fire on the sea monster. Their missiles burn away a coating of much and seaweed, revealing a Megadeus underneath. The robot retaliates at this point, firing missiles of its own, but the MPs are saved by the Big O. Roger and his Megadeus are much more powerful than the sea titan, and he subdues it quickly. Angel begs him not to destroy it, because of the memories it may hold, but he knocks its block off anyway. Back at Paradigm HQ, Rosewater bemusedly remarks that Roger was a little cruel to destroy such a weak robot.
The next episode is Missing Cat.