Zack's girlfriend was kidnapped by aliens and so he embarks on a quest to rescue her. While the story does not play a big role during the game, it is progressed through narrated cutscenes using a comic book style. Zack Zero is a traditional 2.5D side-scrolling platformer: Zack moves from left to right (in some levels there is a second background layer Zack can switch to), jumps over holes and fights enemies with melee or ranged attacks. There are also bosses to beat. Levels have a linear path to follow, but the player is encouraged to explore in all directions to collect all items.
The game's specialty is Zack's suit which allows him to use abilities based on the elements fire, ice and earth. Those are both used to deal with enemies (e.g. an earth areal attack) or help with platforming challenges (the ice power allows to briefly hold time), but also to solve basic puzzles. The powers are automatically upgraded when collecting enough gems which are hidden around the levels.
There are four suits Zack can switch between. The regular suit provides a blade used for both melee and blade attacks. In the fire suit Zack is faster. He can throw a line of flames, surf on fire after double jumping to float, and create a large explosion. The ice suit allows him to slow down time, making fights easier or getting passed moving hazards. With this suit he can also launch a freezing tornado. The rock suit can be used to punch, launch a line of rock spikes or cause an earthquake. While wearing the rock suit it is not possible to jump. Instead, he can break through fragile surfaces and it is also the only mode to pull levers.
At the start of the game Zack has access to all powers of all suits, but after a difficult encounter he is stripped of most of them and has to earn them back gradually by collecting gems. The regular suit can be used at all times, but the other modes drain a meter in the top left corner. The meter needs some time to recharge and in the meantime Zack automatically switches back to his default suit. Defeated enemies leave behind small fragments that add to the score. Levels also have optional secret treasures.
Development on the game was started in 2007, originally with only two people: Carlos Abril and Alberto Moreno. In late 2007 Carlos' brother Ignacio Abril joined the team. The initial idea was to create a 3D game with a 2D structure inspired by Carlos' first commercial 8-bit game Game Over II released in 1987.