Tear Jerker dosen't begin to describe it.
If there is one MLP fic that makes you cry, this is that fic.
Based off of this this comic◊
it tells the story of a despondent young man in a run down city. The set up is something of a cliche, MLP is the only silver lining in the dark cloud of his life, but thankfully the story doesn't go overboard into an emo wreck.
One day he finds a box with a filly Rainbow Dash inside. And I mean filly
, no Cutie Mark or anything. Due to an accident which is explained later, she has arrived here in our world with no idea who she is or how she got here. Our protagonist, for lack of options, opts to take her to his home and care for her, until she can go back to wherever she came from.
The commitment to taking care of his little Dashie stretches out into a 15 year odyssey. She grows and matures in a realistic way, and develops a bond with our protagonist. The heartstrings are first tugged when she calls him "Daddy." Now the emotional aspect is on a whole new level: she has effectively become his daughter. In any other story this would be so laughable it would be hard to take the rest of the fic seriously, but here it's done so well that you instantly accept it. "Yeah, Rainbow Dash is his daughter." You don't even think about it, it's just so...right.
But all good things come to an end. One day Celestia and the Mane 6 arrive to take Dashie home. Dashie doesn't remember them due to the method of her arrival, but she is told that she needs to return to Equestria. What follows is a parting between father and daughter that is so moving even Celestia herself cannot contain her emotions. It's incredibly heart-wrenching, and the punch is amplified when Celestia erases almost all trace of Dashie's existence. Save of course for a note to her father, stating that she will always remember him and always love him, no matter what.
Going by the premise alone, this fic shouldn't work. But it does. It's a heartfelt tale of father and daughter, and learning to let go of the ones you love. Aside from occasional grammatical errors, it's well written through and through. And the final paragraphs are so heart-wrenching that you will not only be in tears, but you will do anything...absolutely anything...to stop Dashie from leaving.