the other games. In between levels the player travels on a two-dimensional map, viewed from above; from the map the player can enter levels by approaching the entrance. Some levels are optional and can be bypassed, while others are secret and can only be reached by following specific procedures. The episodes all contain a different set of enemies in their levels, which the player must kill, stun, or avoid. These enemies are typically aliens or robots. Levels can also include hazards, such as electricity or spikes; touching a hazard or most enemies causes Keen to lose a life, and the game is ended if all of Keen's lives are lost. Invasion of the Vorticons features a raygun that Keen can use ammo for to kill enemies; Keen Dreams exchanges this for flower power pellets that temporarily stun enemies when thrown, episodes four through six use a stun gun that permanently stuns enemies, and the 2001 Commander Keen has a stun gun that temporarily stuns enemies unless they are then killed with a pogo jump. The player can also find food items throughout the levels which grant points, with an extra life awarded with enough points. There are also colored keycards which grant access to locked parts of levels, collectable items that award an extra life beginning with episode four, and items that grant an instant extra life. Plot In the first episode, "Marooned on Mars", eight-year-old Billy Blaze, a child genius, builds a spaceship (the "Beans with Bacon Megarocket") and puts on his older brother's football helmet to become Commander Keen. One night while his parents are out of the house he flies to Mars to explore; while away from the ship the Vorticons steal four vital components and hide them in Martian cities. Keen journeys through Martian cities and outposts to find the components, despite the efforts of Martians and robots; the final component is guarded by a Vorticon. Keen returns to Earth—discovering a Vorticon mothership in orbit—and beats his parents home. In "The Earth Explodes" he travels through the mothership and disables its weapons, at the end discovering that the Vorticons are being mind-controlled by the mysterious Grand Intellect, who is actually behind the attack on Earth. In "Keen Must Die" he fights through the cities and outposts of the Vorticon home planet to reach the Grand Intellect, who is revealed to be his school rival Mortimer McMire,