MAINLINE

MAINLINE casts the player as caffeine injected into an exhausted person. The objective is to travel through the bloodstream and reach the brain before the body metabolises the dose. The player has a survival window of four seconds. Killing enemies adds time, while taking damage removes it. There is no on-screen timer; instead, the screen darkens and the heartbeat sound speeds up as time runs low. Movement is constantly forward through the vessel, with the player strafing around its walls to avoid enemy fire and clots. Each kill increases movement speed, and these increases stack, widening the field of view and lengthening the speed lines. The speedometer is measured in nanometres. Every enemy has an internal weak point, and killing one through that point refills the current magazine.

The game uses a roguelike structure. On clearing a stage, the player picks one of three cards, which either swaps the current gun or upgrades it. Only one gun can be carried at a time, but upgrades to magazine size, damage, fire rate and reload speed continue to apply after switching weapons. There are six chapters, moving from a peripheral vein through the heart, lung, aorta and blood-brain barrier to the brain, with the colour palette shifting from dark venous red to the brighter tones of nerve tissue. Obstacles include adenosine receptors, platelet walls that stop bullets and sweeping clot lasers. A mutant boss guards the end of each chapter. Dying restarts the chapter and removes all accumulated upgrades and the current gun.

Four modes are available: Normal Stages, EX1, EX2 and Endless. The stated expected playtime is one to three hours, and Endless mode results are recorded on Steam leaderboards. The developer permits streaming and monetised video of the game and asks for a link to the store page in descriptions. The game contains heavy use of flashing lights, strobing and sudden colour inversion, and includes a warning for players with a history of photosensitive epilepsy.