The Queen is about to emerge from her private ward at King Edward VII’s Hospital, preceded by her favourite corgis and accompanied by a fanfare of royal trumpters. The stern-looking sister, who is hoisting one of the patients on a pulley, tells the patients, 'Listen ... the national anthem ... she must be leaving! EVERYBODY UP!'
It's traditional for people to stand when the national anthem is played. The joke is that, for a variety of medical reasons, the patients are in no fit condition to stand.
1. A ward is a large room in a hospital, typically one allocated to a particular type of patient: "a children's ward". A private ward is one which is reserved for people, like the Queen, who have private health insurance.
2. A sister is a nurse who is in charge of a hospital ward. (Officially, the title has been changed to "ward manager" to reflect the fact that the role is carried out by men as well as women.)
3. See if you can spot the following items in the cartoon: IV drip, bandages, plaster, medical chart, crutch, stethoscope, sling, neck brace, sheet, pillow, blanket.
'Must' is used here to convey supposition as opposed to obligation. The sister can't see the Queen, but she supposes that she is leaving because the national anthem is being played. • I must be imagining things. See here for a more detailed explanation of this grammar point.
When I was young (a long time ago), they used to play the national anthem at the end of films in the cinema. And yes, people did stand!