Mr. Monk Takes Manhattan - S3-E1
Continuity mistake: In the scene where the two guys are playing chess, the guy with the black hat knocks over the chess pieces after losing. He then starts scrambling up the pieces and leaving his hands near the chess board. When the shot changes to a wide shot of Monk walking away the guy suddenly has his hands on his head.
Chosen answer: By going to the cellar, Donnie has continued to set in motion the events that will return the engine to his proper time. He burns down Jim Cunningham's house, which causes Kitty to be at his trial, which causes Donnie's Mom to take the kids to California, which causes her to take the earlier flight causing the engine to fall back through the portal to the normal universe. In much the same way, his Mom had to leave so they could have a party, which is where Donnie and Gretchen make/fall in love. He takes her to that house to talk to Roberta Sparrow, and it is when he notices the cellar door that the events unfold to motivate him. If Donnie lives, Gretchen dies. He must sacrifice himself to return time to normal so that Gretchen will live. The boys were taking stuff from Roberta's house as referenced earlier in the movie, but played a key role in the events unfolding correctly.
Jazetopher