The final achievement of the game.
While clinging to Edna's DeLorean, Marty needs to attach three "flux overrides": Two on the front and one on top. Move up the right side of the car (select "front bumper") and then use the flux override with the nearby flux emitter. Use the flux emitter and
to aim it at the dish on Doc's car. You don't need constant contact, just a cumulative 7-8 seconds or so. Move back up to the front bumper, then select the front bumper again to trigger a cutscene. Move up to the front bumper again and grab a windshield wiper. Use the wiper to close Mr. Fusion so you can cross over to the left side of the car. Move up to the front bumper and install another flux override the same way as before.
Once Edna rolls down her window, move to the front of the car and talk to her. You roll across to the passenger side, where you can talk to her again. Once you do, she rolls down the passenger window. Now interact with the door to be tossed up on the roof, where you can install the last override. You're upside down but the principle is the same. Enjoy the final cutscenes as our story is wrapped up.