Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

This comic references the real-world controversy in which creationists pushed to have disclaimer stickers placed in biology textbooks stating that evolution is "just a theory." The comic imagines that, following this precedent, existentialists demanded a sticker be placed in every fairytale. The fairytale book reads, "And the good prince married the princess and they lived happily ever after," and the existentialist sticker underneath adds, "But, of course, everyone goes into death alone."

The humor works by drawing a parallel between two groups who want to insert their worldview into texts where it doesn't belong. Just as the creationist sticker undermines the scientific content of biology textbooks with religious doubt, the existentialist sticker undermines the comforting fantasy of fairytales with bleak philosophical reality. Both are technically "true" from their respective viewpoints, but both are hilariously inappropriate in context. The joke also plays on the absurd image of existentialists being organized enough to mount a political campaign for sticker placement in children's books.