He needed a vacation.
For a hundred years he had been terrified to leave Cardiff. But now things were different.
The rest of Torchwood Three had just been put in drawers in the morgue and he felt a strong need for mojitos and chicks who went swimming without their bikini tops on. So like, he needed a fucking vacation ok. And it wasn't like Yvonne was around to tell him no, Jack, don't do that, don't go to that tropical climate, don't sleep with that alien, omg Jack DON'T DO THAT. He was so glad that bitch got converted.
Now he got to go to Mexico, hellz yeah.
He used his Torchwood credentials to get the stupidest fucking job he could find. He was like, a marine safety dude in a boat. Mostly he stole Maine lobsters from the Mexican coral reefs and flirted with the dude on his cigar boat. Shit yeah. He wore jeans that were too tight, to drive the cabana boys crazy. He walked on the boardwalk like it was a fucking catwalk, and he owned it. He thought, "Tim Gunn would approve. Eat your heart out Klum."
So yeah, here he was in Mexico, drawing down a pretty hefty wage to eat lobster, drink cocktails, and have sex. On the beach. It was a pretty sweet gig, overall. He couldn't help but feel, though, that something was missing. He felt the lack of alien activity keenly, and he was getting bored, despite the excellence of the cabana boys and lifeguards.
And then one day, it came from the depths. MEGALADON. Like something out of a nightmare. Jack knew at once it wasn't a normal shark. It was too smart, too fast, too hungry, and most of all - too smart. The moment Jack stared into its dead, beady eyes, he knew the truth. This shark had fallen through a rift in time and space. This was Torchwood territory.
As soon as he saw that shark tooth come out of the low-budget fiber optic cable he knew that they were in trouble. Big trouble. The biggest trouble he had seen outside of the United Kingdom, or really, this side of the Atlantic. He knew it would be up to him to fix this, now that both Torchwood One and Torchwood Three were basically out of commission. He was immortal after all - what could possibly go wrong?
He got hammered, ate his lobster, and posted some pictures of the shark tooth online, using his special Torchwood camera, which he swiped from his former boss's cooling corpse. Immediately some blonde "doctor" replied to his message. She looked kind of familiar, so he assumed that he had seen her in a porno somewhere. She was all like "blah blah Mako sharks," and Jack was all like "yeah whatever, show me your tits blondie." Unfortunately she declined to answer his second message, even though he had typed this one in ALL CAPS.
He decided to cut his losses and continue with the shark hunt. When the blonde finally responded to his message he confirmed that this was Riift activity. Only the Rift, far-reaching as it was, could produce a prehistoric and somewhat alien monster. And when it ate his cabana boy, he was ready to take that fucker down. So he decided to recruit some old dude that kept eye-fucking him, after that Italian creeper refused to listen to reason and shut the project down. Hopefully he would not have to use his newly-acquired submarine-piloting skills.
Jack was deeply hurt by the loss of the cabana boy or whatever the fuck his job was. A bunch of other fuckers died too, but god whatever. So Jack arrived back at the dock with the boring blonde stick and he asked her exactly what was his mind. He wanted to eat her pussy. She just kind of stared at him, and he suspected some kind of lobotomy was to blame. So they went back to his house, watched I Love Lucy, and then fucked in the shower anyway because the Doctor wasn't there to cock-block him. He tried not to stare directly at her boobs when they were doing it, and he imagined his cabana boy in a speedo, somewhere in heaven. He decided that while fighting alien monsters, hotness was just as important as not being dumb as a fucking rock.
The next morning he and what's-her-face got on the boat with the older dude and went out to explode some cables. "Ben," they all said. "Ben, what is the plan?" Well, Christ, he didn't know. He was grief-stricken, and deprived of his usual equipment, and horny to the point of distraction. So this monster could bring it on - he could take it, match it, and beat it at its own game. He did not count, however, on the explosives not having the desired effect. So, he would have to take this mega-shark on, hand-to-hand. He could do that; he was that good. He took on Daleks; he could take on a fucking fish. After all, he was Captain Jack Harkness, and he could do anything.
So he got an awesome little yellow submarine and piloted it around the ocean. He was the Captain, and he captained the shit out that vessel. That creepy old dude was there too, and he was like "JACK. JACK. JACK, WE'RE HERE TO KILL THE SHARK. REMEMBER." UGHHHHHH. So Jack told him to go outside the safety of the submarine, hoping that the inside of a shark's stomach would be enough to shut him up. Then Jack captained the sub right into the shark's mouth and set off a missile.
He climbed onto the raft and put his head near the blonde chick's vag. No one was really surprised when he turned up totally fine, even though he had just been drowned, eaten by a shark, and exploded by a decommissioned torpedo. He was like "MEGALA-WHO?" and everyone laughed. He floated around in the raft with a bunch of other idiots until they all died of starvation, and Jack threw them overboard to be eaten by other, tinier, less alien sharks. Three months later he washed up in Cardiff, Wales, and decided that the world needed him to get back to Torchwood.