The only thing Harry thought was that Sirius Black was in horrible, terrible, dreadful danger, among other synonyms. Specifically, Sirius was being held deep in the bowels of the Ministry of Magic, being interrogated by the Death Eaters. This, Harry assumed, was not something Sirius would do willingly, as being repeatedly tortured and threatened was not a fun way to pass the afternoon, or at least Harry didn't think so.
Unfortunately, he was going to meet people (or more correctly, a person,) that probably enjoy(ed) doing just that. But Harry didn't know about them (or more correctly, him,), or about the consequences of trying to Floo-travel himself out of Hogwarts, in the new Headmistress's office no less. Big Brother—I mean, Dolores Umbridge, headmistress/dictator of the school threatened poor Harry when she caught him in the act, but thanks to Hermione's quick thinking, Umbridge was detained. Violently. By centaurs.
Having returned to Hogwarts from the Forbidden Forest, Harry and Hermione found Ron, Neville, Ginny, and Luna. Now that the six members of the Gryffindor Trio had banded and had rendered their nickname obsolete, it was time to find a much more effective way to find Sirius.
At first they had been stumped, as the Floo plan had failed and apparating was out of the question. Luna, however, found the solution to their problem.
"We fly, of course."