Fblthp, Lost on the Range

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.

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.

If a plotted card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

While you control Fblthp, you can look at the top card of your library whenever you want (with one restriction; see below), even if you don't have priority. This action doesn't use the stack. Knowing what that card is becomes part of the information you have access to, just like you can look at the cards in your hand.

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."