Kellan Joins Up

• WOTC Official Reminder Textofficial rules
  Plot — Plot [cost] (You may pay [cost] and exile this card from your hand. Cast it as a sorcery on a later turn without paying its mana cost. Plot only as a sorcery.)

• WWizards of the Coast2024-04-12
  Official Ruling
  If a card doesn't have a mana cost, its mana value is 0.
• WWizards of the Coast2024-04-12
  Official Ruling
  If the mana cost of a card in your hand includes , X is 0 for the purpose of determining its mana value.
• WWizards of the Coast2024-04-12
  Official Ruling
  Exiling a card using its plot ability is a special action. Once you announce you're taking that action, no other player can respond by trying to remove that card from your hand.
• WWizards of the Coast2024-04-12
  Official Ruling
  You can't cast a plotted card on the same turn it became plotted. On any future turn, you may cast that card from exile without paying its mana cost during your main phase while the stack is empty.
• WWizards of the Coast2024-04-12
  Official Ruling
  The mana value of a split card in your hand is based on the combined mana cost of its two halves.
• WWizards of the Coast2024-04-12
  Official Ruling
  Plot abilities are written "Plot [cost]," which means "Any time you have priority during your main phase while the stack is empty, you may pay [cost] and exile this card from your hand. It becomes plotted."
• WWizards of the Coast2024-04-12
  Official Ruling
  If you're casting a plotted card from exile without paying its mana cost, you can't choose to cast it for any other alternative costs. You can, however, pay additional costs, such as kicker costs. If the plotted card has any mandatory additional costs, those must still be paid to cast the spell.
• WWizards of the Coast2024-04-12
  Official Ruling
  If a plotted card has in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.
• WWizards of the Coast2024-04-12
  Official Ruling
  If multiple legendary creatures enter the battlefield under your control at the same time, Kellan Joins Up's last ability will trigger once for each of those creatures.
• WWizards of the Coast2024-06-07
  Official Ruling
  If an instant or a card with flash is plotted this way, you can still cast it only when you have priority during your main phase while the stack is empty.

Rulings via Scryfall