An army of parasitic monsters is invading several kingdoms. They are led by Zaleya, a sinister leech queen who seeks to enslave the world and force everyone to work as her servants. Eventually, mammals will perish and insects will rule the earth.
Princess Sherry calls upon Sparkster to stop Zaleya, but she also summons his old rival, Axel Gear. The two Rocket Knights are forced to work together if they want to survive.
The game is a 2D platformer akin to past installments of the series, but some segments are playable in 3D.
A new two-player co-op mechanic is introduced. One person plays as Sparkster, and the other uses Axel Gear. Both can emit fire blades from their swords and hang off of tree branches with their tails, as well as charge their rocket packs to reach higher places and bounce off walls. However, they have different capabilities; Sparkster is faster, while Axel Gear is stronger.
New to Valiant Duo is a group of elemental weapons.
- The Pyro Blade emits flames that can be used to light torches.
- The Cryo Musket releases icy wind, which freezes water and enemies.
- The Geo Cannon fires rocky boulders that crush enemies and can be used to push otherwise immovable objects.
- The subtitle, Valiant Duo, refers to The V². "Valiant" has a V at the beginning, and "duo" is a group of two.