A barrier is more than a gateway . . .
Eighteen years ago, Frisk left the Underground, but there was another barrier to face before the monsters were freed. When Frisk comes back as an adult though, she must pass tests that 'destroy her human soul' in order to save the monsters, and Toriel won't allow it. Banished and unable to return, Frisk is stuck back on the surface.
Knowing that freedom can only be won with 'humanity', Sans goes to the surface to trade Frisk some of his 'monster' for her 'humanity', so he can help too. However, neither of them know where these tests will lead . . . or how far the truth really goes.