Most Decrepit Old Bird

If a spell is cast as an Adventure, its controller exiles it instead of putting it into its owner's graveyard as it resolves. For as long as it remains exiled, that player may play it using its primary characteristics. If an Adventure spell leaves the stack in any way other than resolving (most likely by being countered or by failing to resolve because its targets have all become illegal), that card won't be exiled and the spell's controller won't be able to cast it as a permanent later.

If an effect refers to a card, spell, or permanent that has an Adventure, it won't find an instant or sorcery spell on the stack that's been cast as an Adventure.

If you cast an adventurer card as an Adventure, use only its alternative characteristics to determine whether it's legal to cast that spell. For example, if you control Gandalf, Party Guest ("At the beginning of combat on your turn, you may cast an instant or sorcery spell with mana value X or less from your hand without paying its mana cost, where X is twice the number of legendary Wizards you control."), you could cast Burglar's Plot from your hand without paying its mana cost (provided you control enough legendary Wizards), but not Bilbo, Luckwearer.