During the State Opening of Parliament in the United Kingdom, one member of Parliament, known as the hostage MP, travels to Buckingham Palace before the Queen travels to Parliament. The hostage is released upon safe return of the Queen. This tradition began when the monarch and Parliament were on less friendly terms.
The hostage MP is usually a government whip. In 2014 the hostage was Desmond Swayne. In 2015 the role was fulfilled by Kris Hopkins.
References
Hostage MP Wikipedia(Text) CC BY-SA