Ratcatcher Trainee

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 cast it as a permanent spell. 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 adventurer card ends up in exile for any other reason than by exiling itself while resolving, it won’t give you permission to cast it as a permanent spell.

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 Johann, Apprentice Sorcerer (“Once each turn, you may cast an instant or sorcery spell from the top of your library.”) and Questing Druid is on top of your library, you can cast Seek the Beast, but not Questing Druid.