Hermione had deduced that the Chamber was likely in Myrtle's bathroom.
And it was. They noticed a carved snake.
And then they learned Harry could speak Parseltongue when he accidentally hissed the question "but how do we open it?"
Since they knew for a fact that Harry himself had just realized he could speak Parseltongue, neither took it as a warning sign. They slid down the pipe, Harry going first holding onto a cage that held a rooster in it. They'd had Hagrid get them one, with the excuse that they were gonna stuff it under someone's bed.
Hagrid might not be a prankster himself, but he'd had a whole lot of laughs during the Marauders' stay and, seeing as how Harry seemed to be following in his father's footsteps, decided to help them out. The giant had his ways of getting creatures.
They passed the giant skin of the beast, Ron nearly shat himself, Hermione had to hold the urine inside, and Harry whistled at the sight. They continued walking until they were in front of a giant statue of a rather ugly man who seriously needed a good shave. Also everything smelled like goat urine for some reason.
Then a voice that had a clear lisp sounded through the chamber. "Master! Master! Where's the master I've been after? Master! Master! Is that you in my lair?"
"That is probably the basilisk. Blindfolds on everybody!" Hermione said, hearing nothing but hisses.
"Wait, let me try something," Harry told her, before thinking of snakes and trying to speak Parseltongue, knowing that the thought of 'talking to the basilisk' was probably all he needed. "I'm your new master," he hissed.
"Okay," it was far away still, but it was getting closer.
"Go back to the chamber and sleep. I will call you when I need you," Harry said.
"Okay," the creature spoke, and then the rumbling stopped.
"I can't believe that worked," Harry said, in a normal tone.
"What did you do?" asked Hermione, tilting her head.
"I thought trying to reason with it was possible, so I tried and… I just got myself a pet basilisk," explained Harry. "It seems it will accept anyone who speaks Parseltongue as its master. Snakes seem awfully obedient."
"Bloody hell…" Ron trailed off, eyes wide.
"You should probably tell it to only accept you or your chosen as masters now. Just in case," Hermione cautioned.
And so Harry did.