MegaMari: Marisa's Ambition is a Mega Man fangame made by Twilight Frontier in 2006 starring Marisa Kirisame & Alice Margatroid. This game is similar to Mega Man 2 but with danmaku for harder difficulty.
Patchouli Knowledge, tired of Marisa and Alice borrowing (and never returning) books from her, stashes her book collection inside a fortress with many traps and gets help from various other Touhou characters to guard the collection. Marisa, aided by Alice, attempts to defeat all the other girls, gaining their unique powers, before finally raiding Patchouli's castle to get her hands on all the books.
For more information, see Touhou wiki page for MegaMari.