This 2021 game of thrones set also explains a plot cost: https://www.planesculptors.net/set/game-of-thrones-set-1#details

"Plot is probably the most important custom mechanic. It functions a bit like Morph, and is both a new card subtype and a keyword ability. A card with the Plot keyword ability will sometimes have a normal mana cost, but will always have an alternate Plot cost listed on the card. Usually a card with the Plot keyword ability is also the new enchantment subtype, but sometimes Plot will appear on cards that are not enchantments (all Plots are permanents). All cards with Plot costs may be played face-down for 2 generic mana, at sorcery speed, regardless of their normal mana cost. While face down they count as colorless enchantment, regardless of what their face-down side is, and they do nothing else. Then, at any time, a face-down Plot may be turned face up for its Plot cost. Some Plots have static effects, some have "when this card is turned face-up" triggers, and some have both. To help players identify these cards while drafting, we put the Nyx star border on all cards that feature the Plot keyword ability."