Time Slip features the adventures of Dr. Vincent Gilgamesh, a scientist attempting to foil an alien invasion to Earth by traveling to different historic ages. The game takes place in 2045, when the Tiamatians, intelligent alien beings from planet Tiamat, discover a sort of space-time portal -or "rift", as it is called in the game- that interconnects their homeworld with Earth. Meanwhile, human astronomers discover the same rift, but are not aware of the existence of Tiamat. They also discover that the rift is growing in a geometric rate. As they try to study it, the Tiamatians launch an exploratory probe, named "Torquemada II", in order to find a planet similar to Tiamat; the probe soon finds Earth and sends information back to Tiamat.
As Earth astronomers continue studying the space-time rift in 2047, the Tiamat leadership agrees to send explorers to Earth. It turns out that Tiamatians are planning to conquest Earth to convert it into "Tiamat II." However, Tiamatians have learned that humans have highly developed fighting skills and a vast amount of weapons, and they obviously would not agree to be enslaved. Because the power of the human defense forces, they decide to change their strategy, abandoning a conventional, frontal attack on Earth forces (because they would likely face a high casualty rate and even a defeat) in favor of a plan focusing on a time-traveling technology. This plan consists in sending Tiamatian expeditionary forces to several historic ages, in order to interfere with the development of human weaponry. By doing this, they can secure a victory by truncating the evolution of human weapons, leaving humankind with nothing but weak weapons and at the mercy of Tiamatian forces in the future. But their carefully planned strategy has a flaw: it failed to take in count the human intellect.
As Tiamat expeditionary forces are dispatched, a group of scientists led by Dr. Vincent Gilgamesh unveils the prototype of a time-travelling machine. Suddenly, Tiamatians launch an attack on the laboratory where Dr. Gilgamesh works; all but Dr. Gilgamesh are killed. Without having time to test the time-traveling machine, he decides to travel back in time to stop Tiamat's conquest plans. A battle between Dr. Gilgamesh and Tiamatians starts, and it is only a matter of time to decide who shall win.
The gameplay is similar to that of Contra III: The Alien Wars, although with simpler moves and nine lives, but no continues (as opposed to most games of this genre). The game's mechanic also features a TGS bar, or "power bar", which must be filled with special time crystals before emptying; otherwise, the player will lose a life each time the bar empties. The TGS bar is a part of the time-traveling device, and must be filled frequently. The bar slowly empties itself and by being hurt by an enemy, so players must literally "run for their lives" and collect as many crystals they can. After scoring a given number of points, players are awarded extra lives.
Levels are long and full of enemies and traps; players may find themselves fending off lots of weak enemies before battling an oversized (albeit weak) sub-boss, only to face an even stronger, bigger level boss. Traps, in turn, are based on the technology available in each age. A common occurrence was to mistake a sub-boss for the level's boss, because both sub-bosses and level bosses always were oversized and strong.
Regular enemies include humans from past ages and Tiamatians; they often appear together. The game also features animals and mythical beings. Sets of traps are scattered all over the game, but are quite simple. Bosses and sub-bosses often take the appearance of oversized mythological figures, always adjusting to the age they appear on.
The player can use several weapons as laser guns, bombs, etc., which are collected as the game progresses. However, the player can lose its weapons after losing a life. It is also possible to use armored vehicles in some levels.
Unlike other games, there is no cheat code that extends the amount of lives, nor can the player adjust the difficulty level or the number of lives to start with. The game also lacked of continues or a saving mode, forcing the player to restart the game from the first level after running out of lives. After running out of lives, the game ended and a screen appeared announcing Dr. Gilgamesh's fate by ominously declaring "You Failed. You Was Never Born. Earth Becomes Tiamat II" (implying that the Tiamatian plan succeeded and that also intervened on Dr. Gilgamesh's life), as he fell into a whirlwind-like vortex.
Source: Wikipedia, "Time Slip", available under the CC-BY-SA License.