Spoilers: Some for season two episode twelve, "Fragments". Also contradicts the backstory set up in the novel "Another Life".
Disclaimer: Definitely not the owner of Torchwood, though it's run in pretty much the exact same way I would run it.
Notes: I don't speak French fluently, so I sort of pieced that titular phrase together (it means a slow death). As such, the arrangement of words and the gender I used may not be correct. Do tell me if I've done it wrong (or if I've done it right, I'd like to know that, too) if you speak French.
Marriage. It was the kind of word that Owen Harper's mouth was unused to saying; his lips unused to the feeling of the word that they were forming, now, almost every day.
He had met Katie (the love of his life, and the woman of his dreams) such a short time ago (or, at least that was how he saw it), and now they were getting married. She was a junior doctor in the hospital where he worked, and even though the first time he laid eyes on her she was covered in a patient's blood, he knew that he would love her.
They were engaged after a year (seemed like not so long ago they went on their first date), ready to be married in mere months. To Owen, the planning seemed to go on for far longer than their actual courting. But he had just supposed that they were so in love (so eternally connected) that it was right for them to be moving so fast.
Then Katie began to forget.
It was only simple things at first. She would misplace her car keys (something all normal humans do) and wouldn't be able to remember where she had put them without his help. Then she forgot the difference between the brake pedal and the accelerator, and drove the two of them (while on their way to a meeting with the wedding planner) into a solid brick wall.
After that, she went from highs to lows like she was flying. One moment she could be perfectly lucid (and know that she loved Owen; always had and always would), the next she would barely be able to remember the name of the hospital where she worked. And that terrified Owen (the senior doctor, better than her, more knowledgeable) because he couldn't find the cause.
Alzheimer's was obvious (harder to pronounce than marriage, too, and especially unfamiliar) but Owen wouldn't believe it. She was too young. She was too beautiful. She was too perfect. She couldn't possibly have something like that. And after looking closer (so close that his nose was pressed to the scans, and he could see them dancing before his eyes while he slept), it was blatantly obvious that she didn't.
So they stopped looking, and Owen lived with her slow mental deterioration (and his increasingly speedy wish for the answer to her problem).
It was so close to their wedding date when they finally detected a tumour (or so everybody thought). And so they operated, and then the man in the blue greatcoat appeared, and suddenly Katie was dead, and Owen was broken.
Marriage was no longer a necessary word to pronounce (in fact, it hurt him whenever he did say it). The funeral was long, and difficult, and was held the day after they were due to be wed (the same flowers in her bouquet were scattered on her casket as it was lowered into the grave).
Owen didn't sleep that night, his dreams plagued by Katie and her perfectly normal scans and her perfectly beautiful face.
At the cemetery, so soon after, the man in the greatcoat appeared again and offered Owen a job (catching aliens, no less). Owen had no choice but to take it, or be plagued with the guilt of losing her (even though it wasn't really his fault), and the trauma of seeing (and operating in) the very room in which he watched her die every day of his life.
Death. Torchwood. Aliens. Impossible notions to a now broken man who wanted nothing more than to marry his sweetheart and live happily ever after.
Owen's life was crumbling, and he couldn't find a single way to fix it.