This cover is paying the 25¢ airmail rate from Vancouver, BC to Christmas Island (Kiritimati) in the Central South Pacific. Dated November 4, 1957, it was mailed to an officer in the Royal Engineers and is backstamped November 7th, just three days later! The addressee would have been there as part of the contingent that were conducting H-bomb testing at that time as part of Operation Grapple [see https://pacificnukes.wordpress.com/uk-vets/ and also https://en.wikipedia.org/wiki/Operation_Grapple]. The British detonated a 1.8 megatonne device on the island the very month this cover was delivered. I've also included a couple of photos that I found online of the British Engineers doing their work on the island that year.
British Engineers working on Christmas Island in 1957.