At some point in the future, humanity has colonized the Moon and Mars. However, a 20 trillion ton asteroid then Colony Drops Earth, killing off the rest of humanity and making the planet uninhabitable. You are the commander of Moonbase, a small self-sufficient colony. The base has a number of facilities: habitats (more can be built), power generation (better reactors improve efficiency), production (for building pretty much everything), resource extraction (always active given enough power), research (discovering new tech and analyzing probe data), defense (controling Attack Drones and Kill Sats defending the colony), hangars (for outfitting, crewing, loading, and launching ships and probes), and storage (holding resourecs and/or certain items in special shelters that survive attacks). One of the first tasks is building and sending probes to other planets and moons in order to determine their viability as colony sites. It's worth noting that only Moonbase is self-sufficient. The other colonies are significantly smaller and lack any production, research, or storage facilities. They are only used for mining, and the resources must then be shipped to the Moon by transport ships. The Asteroid Belt can also be mined by special ships. But beware! The Mars colony believes itself to be the superior surviving colony of humankind, and will do their best to eliminate the Moonbase when it reaches its fingers out to the rest of the Solar System.
You can build several types of ships on Moonbase: Probes (unmanned, used to scout planets, moons, and asteroids), Grazers (medium-sized ships used for asteroid mining), Waveriders (small, fast ships used to quickly go between colonies to deliver supplies and a small amount of minerals), Carracks (large, slow ships used to deliver a lot of supplies and can carry a large amount of minerals), S.I.O.S. (single-use colony ships; even slower than Carracks), and Fleet Carriers (Martian-designed Attack Drone-carriers that allow one to strike another colony). There is no currency in the game, although each colony needs sufficient power for operations and various resources are needed to build anything.
In 1991, Ian Bird made a sequel for Amiga and Atari ST called Deuteros: The Next Millennium that takes place 800 years after the end of the first game with Earth as your primary world and much of the technology of the first game forgotten (including space travel). The game mechanics are very similar, although, eventually, Faster Than Light travel will be discovered, allowing humans to travel to and conquer other star systems. Additionally, Deuteros is much more difficult than Millennium.
An unofficial remake has been made that updates the graphics and allows the game to be more easily run on Windows.
The game provides examples of:
- Adam and Eve Plot: At the time of the Colony Drop, Moonbase population is about 200. Given that one of the tasks is the total destruction of the Mars colony, that's the breeding stock for repopulating the Earth. Given what we know about genetics, 200 people (even if we assume they're all unrelated), would not be enough to provide a stable long-term population, and humanity would eventually die out. Then again, we have no idea how long humanity will last past the end of the game. The sequel starts 800 years later, which is not enough time for inbreeding to cause any major damage. Genetic engineering may allow humanity to avoid these issues, but that's not one of the available technologies.
Some text, though, indicates that the population number shown in the game is actually the number of able workers, so the actual population number could be higher.
- Asteroid Miners: Grazers are medium-sized ships that are sent on mining runs to the Asteroid Belt. Occasionally, you may get a message from one that finds a particularly valuable asteroid and asking if it should mine it. If you don't reply shortly, the Grazer will move on. Each mining mission lasts for 5 runs, after which the Grazer crew will ask for more instructions. Naturally, you can send them on another mining mission.
- Asteroid Thicket: When viewing Grazers sent on mining runs, their screen shows several large asteroids close together. Additionally, the first probe to be sent to the outer Solar System will usually get destroyed in the Asteroid Belt due to this trope. The text mentions that future probes and ships will be sent above or below the belt to avoid any collisions... without any change in travel times.
- Attack Drones: All fighters are AI-controlled. You can research tech that improves their combat qualities.
- But What About the Astronauts?: Averting this is the whole point of the game.
- Casual Interplanetary Travel: Partly played straight. Traveling within the Solar System is much faster than in Real Life. Nobody has to worry about supplies. On the other hand, stellar bodies move, so distances and travel times depend on their current relative positions.
- Colony Drop: Not long after establishing colonies on the Moon and Mars, Earth is hit by a giant asteroid that renders it uninhabitable. It's later revealed that some plants and insects survived.
- Death from Above: If your defenses aren't enough to destroy the attacking fighters, your colony will be strafed by them, resulting in a lot of damage and many casualties. A large enough attack can depopulate a colony.
- Energy Weapon: Fighters and Kill Sats are armed with lasers.
- Kill Sat: Once you start getting supplies of a rare resource from the only planet/moon that has it, you can build laser satellites that are powerful single-shot defenses that can obliterate a number of enemy fighters before they even get close.
- Mutants: non-vacuous planets and moons usually cause any colonists to mutate into Human Subspecies. At some point near the end of the game, these are the first to secede from Moonbase control (despite the fact that they lack any production capabilities). The Martians who keep attacking you are mutated colonists.
- The Plague: After you have established mining bases on several planets/moons, one will suddenly have its population start dying from a deadly plague. With the medical data transmitted from them, you can research and manufacture a vaccine that must then be delivered to the colony before everyone is dead (likely via a Waverider, since they're the fastest ships capable of carrying cargo). Eventually, every other colony will start to suffer from the same exact plague. You can prevent that by sending the vaccine there before the plague starts. If you don't deliver the vaccine soon enough, the colony will have to be repopulated (the best way is to send a Carrack there and then sends it 80-person crew to the colony pool).
- The Right of a Superior Species: The mutated colonists on Mars believe themselves to be a supreme race and claim all of the Solar System. They are more than willing to destroy you if you don't comply (which you won't).
- Scripted Event: There are a plenty of crises that will unfold almost the exact same way every time you play, some of which are mentioned above. For example, the first high-powered solar power generator you build will always explode due to a design flaw. Once you subdue the Martians and enter endgame, the game hits you in quick succession by several crises, including as a huge Martian retaliatory strike and your colonies proclaiming independence.
- Sequel Difficulty Spike: Deuteros is much more difficult than Millennium.
- Taking You with Me: Mad with grief, the last Martian admiral spends everything to (successfully) depopulate the Moon Base with a Suicide Attack, with the settlers crashlanding on Earth being the last survivors. Also plot point of Deuteros the sequel. The player rediscovers the moon base's ruins and guesses humanity must have been here.
- Terraforming: Restoring Earth to its former state is the final goal of the game.