So aliens come to visit Earth in a giant mothership, but someone accidentally activates an onboard superweapon, stripping the Earth of its water and atmosphere and immediately killing all life on the planet except parasites and microbes. The whole ship is deactivated just before it can start fucking nuking the planet too.

So one alien comes down to the surface a few hours later and finds a dead human who happens to have a spacesuit on. He uses a special device to shrink himself down to fit inside the human's digestive tract, then comes back to the ship with the human's corpse and some disgustingly enlarged worms and bacteria.
Somehow a gigantified amoeba becomes a comedic relief character.
The alien also acquires a living cell from the human's body, and must preserve it, clone it, and ultimately bring back the human race to live together with the aliens on his home planet.