Basic Trope: A good leader is secretly replaced by an evil lookalike who then abuses his position until the heroes can set things right.
Straight: King Sable begins acting oddly, turning from a fair and just ruler to a cold and cruel man who raises taxes and jails all who speak out against him. People comment that he's not acting like himself lately, but it's not until Albert and his companions arrive and investigate that the truth is made clear: he's been replaced by a doppleganger!
Exaggerated: King Sable, Queen Satine and everyone in the royal court (save their daughter Velvet) are replaced by evil shapeshifted monsters who proceed to turn the once-beautiful kingdom into a dystopian nightmare.
Downplayed: The school principal, who's a Reasonable Authority Figure and He Who Must Not Be Seen, has been tricked into staying home and replaced by a Jerk Ass with a talent for mimicking voices. The fake principal announces that since the students are all horrible little swine, they all have detention.
Justified: The Big Bad is trying to usurp the kingdom's throne by replacing the king with one of his minions. By the time people became aware of the deception, it would be too late to stop him from consolidating his power.
Inverted: A good person secretly replaces an evil leader.
Subverted: King Sable's attitude change is easily explained by having to fend off The Empire.
Double Subverted: His attitude shift is explained away by having to fend off The Empireuntil Albert discovers that he was secretly replaced by one of Emperor Evulz' minions.
Zig Zagged: King Sable has grown harsher recently, so the hero Albert suspects that he's been replaced by a monster working for Emperor Evulz. It turns out, however, that Sable is still the real king; the continued stress and paranoia of fending off the empire has led him to drastic measures out of neccessity. Later, Emperor Evulz takes advantage of this to slip a doppleganger into the court — not for King Sable, but as Queen Satine, who encourages her husband to even greater lengths in the name of defending their land.
King Sable isn't kidnapped and replaced with a monster.
King Sable really is evil the whole time.
Enforced: King Sable, as a Reasonable Authority Figure, limits the opportunities for drama, so the writers want to replace him in a Tyrant Takes The Helm storyline. Unfortunately, it's an extremely low-budget Live-Action TV series, and they can't even afford to cast a new actor for the role. Or King Sable's actor's contract stipulates that they can't so much as give him paid leave. They have to think of some way to have the same actor play a more villainous role.
Lampshaded: "I might have managed to fool the people into thinking I'm the real King Sable, but that doesn't explain why they're putting up with my corrupt rule instead of rebelling."
Invoked: Emperor Evulz uses his Black Magic to create an evil monster in King Sable's guise so that he can replace him and bring the kingdom down from within.
Exploited: King Sable's younger brother argues that all this "evil doppelganger" hogwash sure is convenient and maybe he should give up the throne to someone who's not going to go over all "evil doppelganger" on everyone.
Defied: Emperor Evulz refuses to consider such a plot, as it would eat up too much time and resources that he feels would be better applied to other pursuits.
Discussed: "So King Sable has been acting very strangely lately, and has been acting the very opposite of how he normally acts. I'll bet money someone's replaced him."
Deconstructed: Even after the fake king is defeated, the damage done to the real King Sable's reputation and the kingdom remains. Political opponents decry the oh-so-convenient "It wasn't me! Just a monster that LOOKED like me!" defense, especially if the monster was slain inside the castle and away from the public eye.
Damaging his reputation and weakening the kingdom was the entire point; despite the difficulty, King Sable works tirelessly to restore his land to its former glory, reaffirming his love for his people and his family in the process. Ultimately, by the end of the series, the kingdom has grown stronger for its ordeal.
Aware that the "It was a fake!" defense won't work without proof, the Genre Savvy Velvet ensures that the heroes confront, expose and defeat the fake king out in the open so that the public can see the monster themselves.
Played For Laughs: Fake Sable is a complete bumbling idiot who has to ask Princess Velvet, Queen Satine, and Albert their names every so often, and stalls for time when he can't remember in the middle of a conversation. He also consults Voice Impersonation For Dummies when talking. The guards, having spotted the thread, beat the real King Sable's location out of him, rip off his mask, and stick Fake Sable in the Stocks for Royal Impersonation.
Played For Drama: King Sable and Queen Satine, once fair and kind rulers, have gradually grown colder and crueler and taken measures they never would have seriously considered before, such as jailing all who disagree with their decisions and proposing an alliance with Emperor Evulz, even going so far as to offer their daughter's hand to him! Velvet suspects the worst, but due to her position is unable to investigate as much as she'd prefer, so she recruits Albert and his band to ferret out the truth — hopefully before it's too late.