Rupert and the Mystery Isle is the 50th episode of Rupert and the 11th episode of season 4.
While on vacation in Rocky Bay, Rupert sees an island that shouldn't be there. His old enemy, Sir Humphrey Pumphrey, is at the helm of an elaborately disguised submarine, intent on plundering King Neptune's Palace.
- The first and only appearance of captain Barnacle
- The second and last appearance of Sir Humphrey Pumphrey
- Rupert wears the exact same diving suit as in Rupert's Undersea Adventure.