This is a hack of Metroid: Zero Mission that removes the exploration element of Metroid to focus on the boss combat. The game begins in a series of hallways where Samus can collect any and all of her power-ups. Following this are 3 different paths that lead to the 3 different ways you can fight the bosses.

Fight all the bosses in sequence without any opportunity to heal
Fight all the bosses in sequence with the option to heal between each fight
Fight the bosses in (almost) any order you choose

More information on the features and quirks of this hack can be found in the readme.