In this game our main mission is to stop the Collectors( aliens who are kidnapping entire human colonies.) Collectors are working for a sentient race of machines the Reapers( Reapers destroy all advanced civilizations every fifty thousand years.) In the beginning of this game Collectors attacked and destroyed our ship Normandy. Our main character Shepard died in the process of the attack. Secret pro human organisation Cerberus and their leader Illusive Man somehow brought us back to life and build as a second Normandy. Now we are gathering the team of most badass scientists, soldiers and mercenaries to save galaxy and humanity from Collectors and the Reapers.
In the last part we used Omega 4 Relay and traveled to Collectors base. We destroyed the base and wiped out the Collectors but Reaper threat remains real and imminent. We also finished the main game and slept with Jack in last part.
In this part we did a favor to Admiral Hackett. His friend Dr. Amanda Kenson was captured by batarians. So admiral asked us to go on a solo mission and rescue Kenson from batarian prison. Apparently Kenson has vital evidence about upcoming Reaper invasion. We saved Dr. Kenson from batarians. She told us that she is part of the project that aims to destroy mass effect relay(and as a result also a star system) to delay Reaper invasion. Also the Projects members found some Reaper artefact. This artefact indoctrinated(brainwashed) the Projects members including Kenson into believing that Reapers are coming to save us. So the Project's people were not gonna destroy the Relay and Reapers were suppose to be arriving only a few days. We wanted to destroy the Relay and delay the invasion. But Kenson and her minions captured us. We managed to free ourselves but it took us nearly all the time we had left. After that we set an asteroid to clash with Relay and destroy it. Dr. Kenson and her army of lackeys tried to stop us and blow the asteroid before it could hit the target. At this point Kenson was completely brainwashed and wanted to serve the Reapers. So we shot her dead. After that we tried to escape from the asteroid before the collision and explosion. During our escape we saw a hologram of Harbinger( leader? of the Reapers). It claimed that despite sacrificing a star system and batarians we are still doomed. We told him that we will defeat the reapers no matter the cost. After that our communication ended and our pilot Joker arrived with the Normandy to save us. Asteroid hit the Mass Relay and destroyed it( wiping out batarian colony). Then we met with admiral Hackett and told him that his friend was brainwashed and went crazy. Also our character insisted that he did the right thing by destroying that system and batarians. It gave us some time to prepare to fight the Reaper invasion. Admiral agreed with us but said that remaining batarians will not forgive us so at some point we will have to answer for what we have done.
