if the wall behind is cracked, then it suggests a bit of settlement has taken place over the (many) years. Nothing unusual in that.................just patch the plaster if it's not too bad - otherwise as beejay says......replaster the affected area.
The important thing to remember with settlement is that it isn't so important that it's taken place - that's quite usual (historic)............ but that it isn't still happening......... just keep an eye on it over time.............