Set in a time when humans are another species in the intergalactic melting pot, Commander Shepard discovers a massive threat in the Reapers, a group of sentient machines who emerge out of nowhere every 50,000 years to essentially wipe the slate clean by destroying all organic life in the galaxy.
Naturally, Shepard and friends weren’t fond of the Reaper’s plan and set off on a mission to stop them. While on this mission, Shepard resolves disputes, discovers the nature of the ancient Protheans, interacts with different alien races, defeats the extremist group Cerberus, and breaks fraternisation rules by romancing crew members. After the final battle, Shepard’s final choice and eventual death will lead to one of the most divisive game endings ever made.
TL;DR: Shepard saves the universe from machines while sexing up aliens.