For Remus, Peter would be the only evidence substantial enough to exonerate Sirius.

For Harry, the rat would, in one way or another, ensure his parents get justice. Even if partially so. Either he betrayed his parents, or he confirms Sirius betrayed them.

"I have an idea," Hermione said, as Harry discussed it with her and Draco, "but involves breaking a dozen of school rules, so professor Lupin cannot be involved in it."

"What is it?" Asked Harry.

Hermione explained to them her plan: they use Polyjuice potion to disguise themselves as Dean Thomas and Seamus Finnegan to get closer to Ron, and once they do that, they can retrieve the rat. Alive.

"The potion you're talking about is a very advanced potion, and it might take up to a month to prepare." Draco countered.

"Yes, and some of the ingredients will be hard to find." Agreed Hermione, "but unless one of you comes up with an alternative plan…"

"I agree with Hermione," said Harry. "We need to act while we still have eyes on the rat."

And so, they got to work.

First, Hermione got a written permission from Lupin to borrow an advanced potions book from the Restricted Section of the library. She gave him minimal information.

Second, Harry snuck into Snape's ingredients cabinet while Draco, his favorite student, distracted him.

Once the ingredients were secured, they needed a space to brew the potion. After last year's events, Myrtle's bathroom was off bounds, which was great! They could brew their potion there with no one snooping around.

Hermione, being the brilliant witch she is, along with Draco, who seems to excel in Potions, did most of the brewing, while Harry provided the both of them with moral support.

A month later, the potion was ready.

"We need DNA." Draco noted.

And so, they devised a plan to get that DNA. With the help of Neville, who also got minimal information, Hermione was able to sneak into the Gryffindor dorms, and steal a hair of each Thomas's and Finnegan's pillows.

Once they had all the ingredients, including the DNA, they were ready to execute their plan or, as Draco called it, the rat-trap operation.