He's thinking about her, riding home. Thinking that he's failed her again. That it's the last time it will happen. He'll say goodbye to her tomorrow, and he won't come back until he's proved himself. He finds he's looking forward to the opportunity, to starting over somewhere new, out of Archie's shadow and away from the aunts' watchful eyes. It's started to rain and Brutus is uneasy; he tosses his head and Charlie lets the reins slip a little.
The road is slippery and very dark, but Charlie's anxious to be home and dry and he lets Brutus canter. He must tell the Chief not to let anyone ride the beast while he's gone: he'd be the death of Will or Geordie. Brutus was bred to run and his strides eat up the night, and Charlie's thinking of Rose and he doesn't try to steady him. They're almost to the bridge when Brutus stumbles.
Charlie keeps him upright, barely. He's drunker than he thought he was, so drunk that Brutus nearly has him off. The thought makes him shiver. The way his luck's been running, Brutus would go home to his dry stable, and leave Charlie here to spend the night in the road. But a stride later, he realizes something's wrong and pulls the big horse up.
When he lifts Brutus's hoof, he swears a little. It will be a long wet walk after all: the shoe's twisted half off where Brutus caught it, and even if Charlie can manage to get it flat again he won't be riding Brutus again tonight. The horse turns his head to see what Charlie's doing and Charlie strokes his damp, steaming muzzle and loosens his girth before he leads him home.