Remus left Hogwarts at the end of the year, and Sirius remained on the run. Remus, Dumbledore and Harry were Sirius' only correspondents, the people who believed in him.

The Triwizard Tournament was being conducted in Hogwarts, and someone had put Harry's name in the Goblet of Fire. On hearing that, Sirius came back to stay in the cave near Hogwarts, where his godson could meet him during Hogsmeade visits.

At the end of the third task, Harry and Cedric were transported to somewhere unknown. The moment Dumbledore informed Sirius, he arrived at Hogwarts.

By that night, everyone in the school got to know that Harry had seen Voldemort come back.

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After Sirius stayed part of the night with Harry in the Hospital Wing as Padfoot, Dumbledore told him to go and inform the old members of the Order of the Phoenix who were still out there.

Through that summer, Dumbledore reorganized the Order, new members compensating the loss of so many older members. Though Sirius hated his childhood home, he was glad to offer Number 12, Grimmauld Place as the new Headquarters.

Dumbledore would not allow Sirius to go out and fight, because he was still Wanted in the wizarding world.

The only good thing in it was that Remus often spent a lot of time with Sirius in that house.