Lady Jane Grey was  the de facto monarch of England and Ireland from 10 July until 19 July 1553. 

When the current King, Edward VI lay dying in June of 1553, he nominated Jane to succeed as Queen. But his decision was subverted by the privy council as they declared the dead King's half-sisters Mary and Elizabeth rightful heirs to the throne. But really, it was about religion. Unbeknownst to Jane, she was in grave danger, literally.

Jane was imprisoned in the Tower of London while the Council decided to proclaim Mary as queen instead on July 19, 1553.   
Jane was subsequently convicted of "high treason" and marked for death in November 1553 because she had signed many documents as "Jane the Queen", which she was not, according to the privy council.
She was sentence to burn at the stake, then received a reprieve, BUT ended up being beheaded on the 12th of February 1554, at the tower of London ( tower green), thus becoming, the Nine Days Queen.