Okay, kiddo, have you ever played a game of tag? Imagine if someone cheated in the game and said they weren't "it" anymore even though they really were. That's kind of like what happened in the 1576 plot.
You see, there was a lady named Mary who was queen of a country called Scotland. But there was also a lady named Elizabeth who was queen of a neighboring country called England. Mary was worried that Elizabeth might try to take over Scotland, so she decided to scheme with some other people to get Elizabeth kicked off the throne.
They made a plan to fake some documents that would say Elizabeth wasn't really the queen and that Mary should be the queen of England too. But someone found out about the plot and told Elizabeth. She got super mad and arrested some of the people who were involved.
Mary was eventually caught because of her involvement in the plot and was locked up in a castle for almost twenty years until she was executed. It's not nice to try to cheat in games or try to take someone else's stuff, right? Same goes for trying to take someone's throne!