So you think you've saved the day and that Shepard and Anderson are going to peacefully die on the Citadel overlooking the Earth, when all of sudden the Magical Thing That Will Stop The Reapers doesn't work and Shepard passes out and and flies up on a magic glowing elevator.
A hologram of a little kid talks to you at the top of the Citadel in space and explains that he's the billions of years old being that created the Reapers. He created this synthetic race of galactic destroyers because he believes that otherwise, all of the galaxy will be destroyed by synthetics because that's what synthetics naturally do (even if you've made a human and synthetic fall in love or made peace in a 300-year-old organic-synthetic war). Shepard just sort of nods his head at this.
Because he brought you to this spot, that means his plan won't work anymore (?) so he asks you to pick three options, 2 of which he admits don't actually solve the "problem" either. You that you can either blow the Reapers up by shooting a tube (which will also kill all your new geth friends in case you were just going to pick that option), take control of all the Reapers and get them to fly away by turning into magic blue space energy (even though "you can't control Reapers" was a major theme of all 3 games), or jump into a laser beam and make everything in the galaxy half-robots with wires and shit on their skin (what?). All three options will blow up all the relays and thus destroy galactic society.
You pick one and the exact same prerendered cutscene plays, but with a different color filter depending on your choice. Joker, your pilot, apparently abandoned you because he's flying the ship away from the solar system before the explosion even happens. Then he crashes on an alien jungle planet and your crew that was on Earth with you not an hour before is somehow on the ship too (also two of your crewmates can't eat normal food so good luck with that). Ham-fisted Adam and Eve reference if you made Joker and the robot lady fall in love and... you're done.
Then after the credits, an old man tells a little kid thousands of years in the future that the entire trilogy was a story.
Then a pop-up asking you to buy DLC.