Black Lagoon, also called Chapter 0, is the first chapter of the Black Lagoon manga. It was originally released in March 2001 (in Monthly Sunday Gene-X's April 2001 issue) as a one-shot, and Black Lagoon became a serialized manga one year later in 2002 as a result of the standalone story's popularity.
The story starts with Rokurō Okajima, a Japanese salaryman, getting punched in the face and getting two guns pointed at him. It is shown that Dutch and Revy, members of the Lagoon Company, are the ones holding the guns. They ask Rokurō about a disk, then Benny is shown telling Dutch that a Philippine navy patrol boat is coming. Before they go, Dutch warns the crew of the Melanesia not to follow him or else his torpedo boat will destroy them. Revy takes Rokurō hostage, and Dutch calls Balalaika, a crime lord who hired him, to inform her that they have the disk she seeks.
It cuts to an office in Shinjuku City, Tokyo, Japan where a meeting is taking place. Members of the company which employs Rokurō talk about what happened to the disk and what to do next since the Bougainvillea Trade Company, a front for the Russian mafia, has claimed possession of the stolen disk. Receiving a report from Jakarta from a local Chinese merchant, Kageyama relays to the board that the Russians have not actually received the disk yet. They agree to negotiate with the mafia while a special force attempts to recuperate or destroy the disk.
Back on the Black Lagoon, in the South China Sea, Dutch is arguing with Revy about why she took Rokurō hostage. Benny joins in, also stating that it was a bad idea. She gets angry and starts shooting wildly at Rokurō. Dutch grabs and holds her until she calms down. The captain takes Rokurō to the top side of the boat and explains to him what they do, assuring him that they will contact his company. Back in Japan, at Asahi Industries, the company's director is shocked to learn from Rock's boss Fujiwara that the salaryman has been taken hostage. Nonetheless, the director decides to continue with Kageyama's plan of allowing Extra Order mercenaries to kill the pirates.
The crew of the Lagoon Company, Rokurō in tow, are shown at a bar in the crime-infested town of Roanapur, Thailand. Dutch is talking to Rokurō about the city's inhabitants as well as the bar's owner Bao and even gives Rokurō a nickname: Rock. Benny starts recalling to Rock about how he was originally at a university in Florida when he made enemies of both the FBI and the mafia. As he speaks about the next part, Revy interrupts with her mentioning how she saved him, advising him to stop talking about old stories. Pouring herself a glass of Bacardi, she makes fun of Rock for drinking beer instead of rum. Rock downs a shot glass, inciting Revy, and the two start a drinking contest. Meanwhile, Dutch is shown talking to Balalaika on the phone. She informs him that the Japanese company has sent someone to kill them, telling the boat captain to change his course to Palawan Island to deliver the disk. At that point, a group of mercenaries break into the bar and start shooting the place up. Thanks to Revy and Dutch's gun skills, the Lagoon Company escape from the bar and head back to their boat along with Rock. Afterward, Storm reports to the E.O. Captain that sixteen and eight of their men were respectively killed and wounded, but the captain merely replies that he is glad that they have an opponent worth facing.
Back on the PT boat, Dutch contacts Rock's company, with Kageyama explaining that Rock is the same as dead to them. He confesses that because of their company's low sales, they planned on selling the disk containing blueprints for nuclear weapons to a war-torn country. Subsequently, the group of mercenaries encountered at the bar are getting on a chopper, learning from "Pretoria HQ" that the Japanese company will pay for their missiles. Back on the boat, Rock is vomiting, so Revy takes him to the top side. Benny tells Revy that something is approaching rapidly; then, a helicopter appears and starts shooting at the PT boat. Revy tries to hit it with an anti-aircraft gun to no avail. To her side on deck, she notices that Rock has put on a safety vest and is going to jump into the sea to escape, so she angrily stops him by tackling him. The two are interrupted by more enemy gunfire, causing the PT boat to swerve away.
The Lagoon crew then notices that the helicopter has stopped shooting at them, with the captain ordering his co-pilots to speed ahead and cut off the boat's path to Palawan Island. Mayer comments that they could easily destroy the ship, but the captain specifies that he wants to have fun demoralizing the pirates. Ahead, Benny and Dutch notice that the helicopter deliberately led them into a narrow channel leading to the island, realizing that their path is cut off by the helicopter at the end. After getting punched by Dutch for panicking, Rock formulates a plan to get back at their opponents. Following Rock's plan, Dutch uses a shipwreck as a ramp to propel their PT boat high enough to fire its torpedoes at the gunmen's helicopter. The salaryman diverts the helicopter's heat-seeking missiles with a flare gun, and the plan works, crashing the helicopter.
At the seaport, the Lagoon Company gives the sought-after disk to Balalaika. In turn, she gives the disk to Kageyama. The Japanese manager then offers Rock to come back to work for the company, but he refuses, remembering that he is the guy who left him for dead. A moment later, after the departure of his former boss, Dutch asks Rock what he plans on doing now, and the relieved ex-salaryman remarks that he has nowhere else to go. Smiling, Revy asks him if he wants to become a sailor, and Rock reciprocates a smile and joins the Lagoon Company.