What makes a television scene perfect is when each of its elements comes together to convey the emotion and story beats intended by the scene in the best way possible, and that scene is placed properly within the narrative to invoke the greatest amount of response from the audience.
The final scene of "Not What He Seems" exemplifies all of the above. At this point on the series, halfway through the second season, the lead characters Dipper and Mabel have learned that their Grunkle Stan (Great Uncle Stan), who they've been living and bonding with all summer, is not who he says he is.
In this scene, Stan has activated a portal in a secret lab underneath their home, the Mystery Shack, for an unknown purpose. Dipper and Mabel try to shut it off as it causes