Sonic the Hedgehog and a human, FRISK, fall down into the UNDERGROUND, a large and out-of-the-way place under a mountain, separated from the rest of the world by a magic wall called the BARRIER.
After a battle sonic persuades shadow to hang out with him and they slowly get to know each-other.
And after the day out they head home but something happens that will change everything
No spoilers
Violence warning