First you need to go to the tower then buy 12 shaders from Evalevante. Then go to the tower hangar and purchase 200 materials with Legendary Marks. After this the stranger will appear where you last saw her in the hangar. She will give you a new quest called "I have...some time to explain..." Upon completing this quest you will have to farm 84 exotic spinmetal, they only spawn once every hour, and if you don't find it in time it will despawn. After collecting the materials, return to the stranger, and you will receive the "No time to explain" Exotic pulse rifle. Good luck!