As Mrs. Western said in Chapter 7 of this book, she showed Mr. Fitzpatrick the letter she received from Mrs. Fitzpatrick.
This letter includes Mrs. Fitzpatrick's address in London.
Unluckily, Mr. Fitzpatrick arrives at his wife's house just as Tom is leaving.
Mr. Fitzpatrick doesn't recognize him at first, but he does demand to know his business.
Tom recognizes him and shakes his hand (because he's a big, dumb puppy, always happy to greet people even when they are clearly bad news).
Mr. Fitzpatrick says that he has been wanting a word with Tom, hits him on the head, and then draws his sword for a duel.
Tom fights back and sticks his sword into the man.
Tom hopes that Mr. Fitzpatrick doesn't die, but he reminds Mr. Fitzpatrick that he has "drawn it upon [himself]" (16.10.9).
A gang of guys standing nearby says that Mr. Fitzpatrick is totally going to die.
And Tom is surely going to be transported for his crimes.
Like the "press-ganging" we mentioned in our "Detailed Summary" of Book 16, Chapter 8, "transportation" was another way that the British Empire forced people into work. It used to be really common for convicts to be sent on ships (or "transported") to the British colonies (including the Americas and, especially, Australia) to supply cheap labor.
Even people who had only committed minor crimes might still get shipped off against their will.