When Harry finally confronts Sirius, he kills him instantly with the basilisk-venom knife before he can tell his story. Lupin arrives and his presence scares Wormtail into running away before Remus recognizes him, so they all assume the "servant" was Sirius, they have successfully averted the prophecy, and Voldemort will not be returning greater and more terrible and all that after all. What a relief! It’s just too bad that Scabbers ran away again, but hey, maybe he’ll turn up eventually.