Story takes place after The Flash (DC, 2016 series) #63, where Barry learned of Wally's death. Wally died in Heroes in Crisis (DC, 2017 series) #1.
Impulse was shown to have returned from the Speed Force after the events of the Flash War in The Flash (DC, 2016 series) #50. In this issue, finding no trace of his old life (pre-Flashpoint), he sets off to find his friends, which he does in Young Justice (DC, 2019 series) #1.
The mystery villain whom Godspeed is working with is later revealed to be Paradox in The Flash (DC, 2016 series) #88.