Warning: This page is chock full of spoilers - the conclusion of this story is revealed, if you're not smart enough to have figured it out yourself.
Mysterious Mare Do Well (a.k.a. Blatant Mare Do Well, Dashfags - The 2nd coming, We Actually Paid for Animators to Make this, Egotistical Rainbows 2, Rainbow Dash's Holiday Special) was a poorly written episode by a new writer, Merriwether Williams, who probably didn't watch all the episodes before writing, and consequently, got the characterization all wrong.
The story opens with Rainbow Dash saving a filly stuck in a well, and is well honored by it. Then she saves a foal in a runaway stroller, which once again, is honored and praised for it. It was that point in time she realized she can become the Element of Loyalty master by turning herself into an arrogant cunt and attention whoring as a "hero" for every chance she gets. Her mindset only gets worse when a masked pony with a cape swoops over and steals her thunder after a rescued parachute accident. Later that day, another runaway carriage is saved by Mare Do Well's massive body strength. Then a bunch of construction workers after a crane failure.Meanwhile at Ponyville's dam, Rainbow realized there was a dam crack, probably due to the lack of dam knowledge from these dam engineers. Which is a shame because that place was a breeding ground for robots to generate dam power. There were a bunch of Transformers to assist with the dam voltage regulation.
When the dam hit its yield point, it instantly engulfed the valley, swallowing Rainbow Dash with it. Lucky for her, Mare Do Well saved her life. Obviously she didn't care. She just wanted to be Ponyville's big superhero again.
Succumbing to a sad realization on a storm cloud that she was a nobody, Scootaloo invites her to town hall for Mare Do Well's appearance at the parade. This is where she finally learns her lesson.
At the end of the episode, Rainbow dash took down the "Mysterious" mare, only to find out it was the mane six all along.