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Tear Jerker: A Study in Regret
Not that the entire fic isn't one big Study in Tear Jerker, but... (Prepare for one heart-rending moment right after another.)
The eternal What-If for Sherlockians: What if Watson had known that Holmes survived Reichenbach? Right there and then? ...Colonel Moran might have shot him in the heart, just as he does in A Study in Regret. Sure, we like to think that if Holmes hadn't done the Great Hiatus, maybe everybody would've lived - even Mary. But the fact that this is a very realistic portrayal of what might have happened if the Hiatus never happened... yeah, cue those waterworks.
Maybe even more heartbreaking than Watson's death is Holmes's guilt over it, described in such vivid detail that it's easy to imagine that, even if he wouldn't get tortured nearly to death, his guilt might well have eaten him alive.
Lestrade calls out "Doctor!" when Holmes wakes up for the first time out of captivity... then realizes what he's done when Holmes is confused at Watson's absence. Then Holmes realizes - Watson's death wasn't a dream, he isn't coming back, Lestrade knows it... The end result is that Lestrade's tears enable Holmes to cry again, and Lestrade holds him.
Mary's very active involvement in the search for her husband and Holmes... only to discover that only one of them still lives. As Holmes's train of thought puts it: Just one of them was left...the wrong one... To top it off, she is pregnant with the child that poor Watson wanted and will never have the chance to meet.
Aside from A Study in Tear Jerker, ASIR could also be called A Study in Survivor Guilt: it's been the defining trait of Sherlock Holmes right up to the present. His best friend died - his best friend who was married and had a baby coming and Watson didn't know it - and Holmes lives on. Who wouldn't be dealing with serious Survivor Guilt and suicidal thoughts? What really sells the whole thing is that it could have so easily spiraled down into irritating Wangstbut it doesn't. Holmes's depression, guilt, and suicidal thoughts are rendered with chilling accuracy. More than that, however, there is just enough humor, enough rays of sunshine, to keep us afloat while we follow the Great Detective through the realistically long and painful process of finding the strength and the hope to live again.
Any time Lestrade or Mary gives Holmes a What the Hell, Hero? talk. Both have valid points but can present them rather bitingly, and Holmes's reactions are just heartbreaking.
Holmes's conversation with Marcel. Marcel is so certain that the detective despises him - and it shocks and horrifies Holmes into his own private self-bashing. It's Woobie to Woobie, and you'd better have a box of tissues nearby because you'll most certainly need them.
And, previously, the scene in which Mary had been the one to hold him while he cried himself to sleep.
Holmes's failed attempt at suicide, and Lestrade talking him down. As said above, it's a scene rendered with chilling accuracy... and this troper was crying a river before she was finished reading.
"Little brother." Marcel is leaving them and it's very much a "Nooo, don't go, Marcel!" moment for both the characters and the readers. They're saying their goodbyes, with Lestrade calling Marcel "son" in German, and then Marcel switches to French with Holmes and gives him the traditional "God go with you." And in replying, Holmes — Sherlock Holmes — calls Marcel "little brother". Tear-jerker and CMOH, doggone it.