Not a complete story, just an idea. Lemme know if this sounds promising. Perhaps more importantly, lemme know if it has been done adequately by others.
An alien spacecraft or entity of immense size and power suddenly appears (emerges from hyperspace, pops in from another dimension, whatever) in the outer-ish solar system. It promptly wanders over to Jupiter and tears the planet to bits, stripping it of deuterium and leaving the rest in a slowly condensing cloud. The planet is chop-shopped down to the rocky core, with an expanding cloud of protium, helium, ammonia and such around it. The Jovian moons all now orbit the sun freely… the ones that the aliens didn’t eat, at any rate. Once done with Jupiter – a process taking at most a few weeks – it turns Saturn into a blinking traffic signal, a light easily visible over substantial interstellar distances. The rings are stripped away, the moons are melting. and drifting outwards. The alien dumps a Mars-mass of fabulously radioactive trash out beyond Neptune and then moves on, back into hyperspace. Doesn’t physically harm us, doesn’t communicate with us, gives no indication that it even notices us… just refuels on a scale we can’t quite deal with. Cultural hijinks ensue on Earth.