Adventure

Adventure mode is a single-player mode in Super Smash Flash, along with Classic mode and All-Star mode. This mode is very similar to the Classic mode, though it includes platforming segments between each of the fights. Though an Adventure mode was confirmed to appear in Super Smash Flash 2, there are very little information besides some outdated posts that appeared on early Smash Flash DOJO!!! pages.

In Super Smash Flash
Adventure mode in Super Smash Flash is essentially a clone of Classic mode, containing the same amount of levels, the same bonus rounds and a similar final boss fight. They differ in that Adventure mode poses side-scrolling levels not seen in Classic mode, in addition that one of the bonus rounds is harder than the one found in the latter.

In Super Smash Flash 2
It was initially confirmed that Super Smash Flash 2, like Super Smash Flash, would include an Adventure Mode based on Super Smash Bros. Melee. According to the DOJO!!! update, each level or world is represented as a mini-planet themed with an associated SSF2-in game universe, such as a Mushroom Kingdom mini-planet from the Mario series. With that being said, this information is outdated, so it may not reflect what actually appears in the final game.

Trivia

 * There are unused hidden files for Adventure mode in SSF2 as of v0.9b that cannot be accessed.