Dipper and Mabel and up back in time and stumble across Ford post Bill betrayal and in the throes of delusional paranoia. He agrees to let them stay in the shack at first, since it's the dead of winter and they claim that the snow is preventing them from getting home (they're actually just stalling for time till they can fix the time measuring tape), but once they start wandering around looking for tools and stumbling across his research, Ford absolutely loses it.
He accuses them of either being shapeshifters or, tiny government agents, or spies working for Bill and goes full tilt ax murderer on them. They try to explain the truth, but Ford is 100% bloodshot eyes, muttering under his breath, past the point of listening to reason. Dipper and Mabel now have to play a deadly game of hide and seek against Ford while trying to repair their only ticket home before he finds them.