There are 3 long games I had to summarise, so obviously I had to omit a lot of things:
- The series is based on the premise that there's a fifth universal force, dark energy, that can be controlled in certain ways to increase or decrease the mass of objects (by generating mass effect fields
, hence the name). This leads to all sorts of applications, like new weapons, shields, faster than light travel and even Force-like telekinetic powers.
- Humanity has only recently discovered this technology and is exploring our galaxy, which is full of other sentient races. The situation is quite peaceful, with a 3-race Council governing from a massive space station called the Citadel. The Citadel wasn't actually built by them, but found, so no-one knows who actually made it and for what purpose.
- The first game starts with you investigating artifacts left by the Protheans, an ancient alien civilisation that disappeared thousands of years ago. You learn that these people were actually destroyed by machines called Reapers for unknown reasons, and that this cycle of extinction has been repeating every 50 000 years. Basically, Reapers come in, kill every advanced organic race and leave the galaxy. Now they're coming back.
- Reapers appear as huge spaceships that are virtually indestructible, so while they can technically be defeated in a "normal" Star Wars-like space battle, it's just not going to happen. There are thousands of them and their technology is far more advanced.
- Turns out the Citadel was actually the key to repeated Reaper invasions - it's basically a giant portal that, if activated, allowed Reapers to teleport straight into the heart of all galactic civilisation and easily destroy everything. This time (in ME1) this plan had failed because the Citadel was sabotaged by the Protheans to no longer respond to Reaper commands, so it had to be activated manually. You defeat the agents of the Reapers who try to reactive the Citadel and all is safe for the moment.
- In ME2 you fight more agents of the Reapers, the Collectors. They've been abducting humans to somehow bring about their "perfection" and "ascension". What exactly they or the Reapers are doing (and why) isn't explained properly, so everyone assumed that ME3 will make their motivations clear.
- The arrival of the Reapers has now been delayed, so they had to resort to a conventional attack - i.e. just fly into the galaxy instead of teleporting into it. This is where ME3 starts.
- You unite the races of the galaxy while scientists construct a spaceship called the Crucible (based on old Prothean technology, once again), a device that supposedly can wipe out the Reapers. Nobody knows how it works, but we do know that it needs to dock with the Citadel. So, at the very end of ME3, the Crucible docks with the Citadel and you (wounded and possibly dying at that stage) reach the Citadel's main control room that has somehow remained hidden for thousands of years. What follows is a nonsensical sequence of events that has given rise to various theories about the ending.
- In the end, an AI that "lives" in the Citadel (which for some reason presents itself to you as a hologram of a small child) tells you that it controls the Reapers. It says that Reaper invasions are necessary because if advanced organic life is not periodically destroyed, then it will create synthetic life, i.e. robots, and then these robots will eventually go out of control and kill everything in the galaxy, making any kind of organic life impossible. This logic is debatable, but in the context of the game it is actually insulting, considering that one of the choices the player can make is broker peace between an organic and a synthetic race.
- The choice at the end boils down to controlling the Reapers - making them leave by somehow fusing with them, destroying them (and all
other synthetic life, including one of your crew members) or creating one race of beings by making everyone into an organic-synthetic hybrid. The implications of your choice are not clear: the AI does not tell you much about each option. Also, nothing is really explained by the cinematics that follow any of the 3 choices and there's a lot of space magic (i.e. nearly magical events that really do not belong in a sci-fi game) involved. The cinematics are also mostly the same, regardless of the choices you make.
- Now, the problem is Bioware has repeatedly emphasised the importance of player choice in all 3 games and placed a lot of emphasis on personal relationships with your ship's crew members. The ending does not explain what happens to your team, humanity or other races of the galaxy. It also makes it seem that your choices leading up to the ending of ME3 were irrelevant.
There's a lot more that can be said about the ending, but essentially it's very unsatisfying to many players and is inconsistent on many levels. Feel free to ask more if you want.