spirits
Explanation
The Joke
This is a promotional comic for Weinersmith's book "Soonish." A character tells another that to become a sorcerer, they must spend a night in the haunted mansion. The second character asks "What? Haunted mansion? The one on Gale Street?" and is told "It's not haunted." The first character clarifies that there are no ghosts -- they just think there might be mice, and it would be good if the second character could check it out, adding "You're extended family" as justification. The other person protests: "No! That part was not accurate!" regarding being called extended family.
The joke subverts the classic supernatural trope of spending a night in a haunted house as a test of courage or magical initiation. Instead of a genuinely spooky trial, the "quest" turns out to be a mundane favor -- checking a relative's house for mice. The supposed mystical mentor is really just a family member trying to guilt someone into pest control duty under the guise of a heroic quest.
The Humor
The humor comes from the bait-and-switch between the grandiose fantasy setup ("to become a sorcerer, you must spend a night in the haunted mansion") and the mundane reality (a family member's possibly mouse-infested house). The final protest about not actually being extended family adds another layer, suggesting the character is more offended by the familial claim than by the deception about the quest. This is a "Soonish" promotional strip, so it plays lighter and more whimsically than a typical SMBC strip.