Gryffindor Tower was evacuated. Sirius thought gleefully, he was on the run being the most sought after escapee in Magical Britain. How could he hide for so long, if he was so desireable(?) you may ask?
Only so many knew this secret, but as half of them are dead, one in hiding and the other a werewolf. The secret was as much as null and void. He was an animagus— he had been one since fifth year. And his form was the grim, a magical dog breed that symbolizes an omen of death. Not like many know that though.
Anyway, he was looking for a man— wait no—a rat. The scoundrel that put him in that horrid prison for so long, they'd thought of him to be dead, that's a lie. Peter Pettigrew, living with a third year Gryffindor, Ronald Weasley. He saw him in the Daily Prophet during the minister's visit and he knew, just knew he was going to get that dirty traitor. He had tried before but things didn't quite go in favour.
That was how he was labelled to have murder 13 people (muggle and Peter) and betrayed his best friends. The only murder that day was probably going to be Peter Pettigrew, though.
Here he was right now, in the Gryffindor common room. The dirty uncared for dog whined in nostalgia at memories that rushed in. For Lily and James he'd thought. As he climbed the boys' dormitories, on the fourth level was the third year dorms, as confusing as that was it was obvious. His dog form creep down the hallway looking at the names marked in shabby plaques. Where he found the one most appealing:
'SEAMUS FINNEGAN'
'NEVILLE LONGBOTTOM'
'HARRY POTTER'
'DEAN THOMAS'
'RONALD WEASLEY'
A few familiar names to him, his godson, Harry, and Neville Longbottom, poor Alice and Frank. Though he wasn't close to Alice and Frank — they were wonderful acquaintances. And he was particularly saddened by the news of the two Order members demise.
Thank god the door was a latch door, his dog form was easily able to use it's paws to open it. When he opened it finally.
He was greeted was the sight of an obviously messy teenager's dormitory. Pieces of parchment everywhere, clothes scattered on the floor. Not surprising. Then he saw his target.
A rat.
Scurrying across the floor, obviously in a rush, missing a finger. Padfoot immediately, growled and pounced on it, trapped in his paw he dropped it down his mouth. Swallowed whole.
That was how a Dog Chases the Rat.