Let's say the team get intel saying London is Null sector's next target, so they fly there in a ship (as you do). The cutscene begins. They arrive too late and null sector have landed and started terrorising the people. They make their landing and head towards the cathedral, which has all sorts of computers in there if you look at the map in the current game. Echo uses these computers to gather info on how to stop the attacks. Meanwhile, Tracer gets a call, It's Emily (Tracer's GF if you didn't know already). She sounds panicked, and tells tracer that she's been found by null sector omnics and have held her hostage with a bunch of other people, but the call cuts out. Tracer is appropriately devastated and has to go rescue her. So Winston, Lucio, Mei and herself set out to find Emily while the others try to stop the attacks. The gameplay follows those 4 as they battle null sector, and track Emily's location. Once the gameplay finishes, the next cutscene happens. We see Emily desperately trying for ward off a tough null sector bot from a family. She is then seen to be knocked into a wall and is about to be killed. Tracer sees her get knocked back and just loses it. Her chronal accelerator gives her insane speed and obliterates the bot that knocked Emily away. They save her, the others managed to stop the attacks, Tracer and Emily share a kiss, the end of that story mission.
TL;DR - Tracer's Girlfriend is held hostage by null sector in Kings Row, so Tracer, Winston, Mei, and Lucio set off to find her.