An alsatian, given unconditionally by the family whose home it has outgrown, is trained as an RAF police dog.
The short film opens with Peter, the dog's first schoolboy owner, sending 'Rex' birthday greetings; the message arrives (RAF training centre, Netheravon) as Rex is completing his training. A flashback shows the dog's arrival, initial hostility, and meeting with handler, and hints at his instruction. Rex passes the final test (halting "an armed intruder") and appears in a display at Peter's home town. Closing image shows Rex on guard by a Valiant bomber, "helping safeguard the nation's security."