Sherlock Holmes had failed.
Not a failure to deduce and act, but in that his client had engaged him to cover his own crimes, not counting upon the detective discovering the truth.
The investigator slumped morosely in the cab amid a cold drizzle, lost in disgust with humanity but especially its gentlemen, who could stoop to such atrocities against helpless women and then have the audacity to employ the Sherlock Holmes to exonerate him from suspicion.
Despicable, the depths to which a man could sink. What was the point? The world would only grow more depraved, and anyone standing against it would fight a losing battle for the rest of time.
The cab stopped. Holmes stepped down, fumbled for his key, mechanically unlocked the door. Realising he was alone, he turned, and perceived the Doctor offering the empty vehicle to a passing young woman carrying a shawl-wrapped toddler and firmly grasping another little one. Watson saw the grateful woman into the cab, then tossed a half-crown to the driver before following his friend into the hall.
The detective felt a smile curve his lips; perhaps there was still hope for humanity.
Holmes clapped his astonished friend on the shoulder with a quiet "Thank you, Watson" before continuing on up the stairs, leaving the poor Doctor staring after him in some bewilderment.