Adventure Mode: The Flash Of Shadows

Adventure Mode: The Flash Of Shadows, often abbreviated as FOS, was the planned downloadable "story mode" extension for the downloadable version of Super Smash Flash 2. Based on The Subspace Emissary from Super Smash Bros. Brawl, it would have consisted of various levels in the style of a side-scrolling platformer in which the player must reach the end of each level while fighting enemies and bosses. Additionally, a story would have been told through pre-rendered cutscenes during and between levels. The story had been developed by several SSF2 Developers known internally as The Flash Of Shadows Writers, led by Suliboy. The story was planned to be about a group of heroes' struggle to defeat a "Shadowy Man" along with his minions known as the "CuBots". Characters would join the player's party according to the story, and allowing unlockable characters to join the party would unlock them.

Due to changes in the staff in late 2010 following the release of v0.6 of the SSF2 Demo, the current story was deemed absurd and obsolete, being scrapped altogether, and the mode itself was quietly scrapped shortly after. On March 23, 2018, developer James Hadden confirmed in an interview that a story mode is now highly unlikely, albeit still possible, to be created for SSF2 in the future.

Cutscenes
As previously mentioned, a story would have been told in this mode through pre-rendered cutscenes during and between levels. Of these cutscenes, three have been uploaded to McLeodGaming's YouTube account, McLeodGamingOfficial, although they are now unlisted. Note that these cutscenes are all depreciated and based on the original story for this mode, which is now scrapped. Also note that changes have been made to the game's character roster following the release of v0.7, with Vegeta IV now removed and Solid Snake and Zero now unconfirmed, and these cutscenes reflect only the initial roster.

Clip 1
The clip starts on the unknown land where Snake laying down from unknown reason.

Clip 2
The clip starts on Pokémon Stadium where and  battling the competition.

Clip 3
The clip starts on Planet Namek where found the Dragon Ball.

Trivia

 * The original logo for this mode is the same as that for The Subspace Emissary from Brawl, though the text has been changed.
 * Though a section for this mode was included in the game modes tab of the fifth Smash Flash DOJO!!!, no updates for the mode were ever posted in it. The tab was later removed with the website's redesign on February 8, 2013.