The morning after the breach, two familiar Aurors arrived at Hogwarts to collect Callum. Tobias wanted nothing to do with a pack of schoolchildren, but Professor McGonagall talked him into letting the party come along, on the condition they stay on their best behavior and do exactly as he and Rafael said. Otto pushed to take the Floo network, but Tobias refused and instead the Hogwarts Express was run out of season, carrying the four of them alone to London.
At Gringotts, the goblin banker let Callum decide who would go down with him. He left Tobias and Rafael waiting in the lobby and took only his friends into the depths.
The Vault was a mess. A ring rested on a pedestal at its center, and papers lay scattered across a desk and all over the floor. Finlay and Callum sifted through them while Otto turned up a backpack in the mess. Among the documents was a lead from some unfinished investigation about a reclusive witch living in a cabin near Lochinver.
Then they heard the words behind them. Avada Kedavra. Their goblin escort dropped dead. The two witches and the boy from the hidden library they had seen with Primrose had come for the ring. Otto closed his fist around it and ran past them toward the cart platform. Finlay was caught by a Crucio and crumpled, but Otto and Caoimhe answered with Immobulus, freezing their attackers long enough to drag him clear and escape.
They burst back into the lobby to shrieking alarms and a bewildered Tobias shouting for answers. Otto's eyes went straight to the great fireplace. He ran for it, the others piling in behind him, and just managed to catch the Floo. He yelled "McGonagall's office!" as Tobias hurled something across the hall after them. The green fire roared up and swallowed them whole.
The party did not find themselves in McGonagall's office, but somewhere else entirely.