Tulkinghorn has been the Dedlock family lawyer since before Sir Dedlock inherited his title.
When Sir Dedlock was planning to marry Lady Dedlock, Tulkinghorn advised against it.
One day, when bringing legal documents to the Dedlocks, Tulkinghorn is intrigued to see Lady Dedlock recognize the handwriting on one of them.
When he goes to investigate the copywriter, Tulkinghorn finds him dead of an overdose.
At the inquest, he interrogates Jo the crossing sweeper, the only person who knew the dead man.
He tells Lady Dedlock the news and observes her reaction.
Tulkinghorn hires Bucket to find Jo.
With Lady Dedlock's former maid Hortense, he confirms through Jo that Lady Dedlock was touring Nemo's haunts disguised as a servant.
Tulkinghorn refuses to find Hortense a job.
Through Smallweed, he blackmails Mr. George into giving him a sample of Captain Hawdon's handwriting and confirms that Nemo was Captain Hawdon.
Tulkinghorn orders Bucket to give Jo enough money to get out of London forever in order to shut him up.
Hortense and Mr. George each separately threaten Tulkinghorn's life.
After he figures out that Lady Dedlock's daughter is still alive, Tulkinghorn menacingly tells the story, without using names, at a Dedlock dinner party. When Lady Dedlock confronts him, he agrees not to do anything with the information as long as Lady Dedlock goes on as if everything were normal.
When Lady Dedlock sends away her maid Rosa, Tulkinghorn decides that all bets are off and plans to reveal her secret to Sir Dedlock the next day.
He goes home and is shot to death.
His funeral is a grand affair which no one actually attends, but every family sends an empty carriage as a gesture of respect.