Transporter spilt into two.
The Klingon half dies in a brawl?
The human half is going to die, but they shoot her up with Klingon corpse juice via the transporter, and the Engineer seems to be back to normal.
Even though the book keeping accounts this woman to be a completely different new transporter clone.
She's a transporter clone of half a transporter clone of the original.
The final product seems to be the half dead granddaughter of the original B'Elanna by some regards, or just her sister too arrogant to accept that she's not who every memory and fibre insists that she verily is.
Kirk in "The Enemy within".
Her eyebrows got a lot smaller after this episode did you notice? Was she plucking, or was there significantly less active Klingon in her makeup after the recombination?
"If Groucho Marx and Brooke Shields had a baby, she would have your eyebrows."
(The Princess Diaries.)