Although Luther possesses incredible strength and durability, a botched mission to the moon leaves him injured, leaving Reginald no choice but to inject him with a life-saving serum.
Unfortunately, the serum also left Luther with the physiology of a huge ape, rendering him a hulking mass of fur and muscle.
The production team used an impressive combination of prosthetics and CGI to create Luther’s impressive shape and size.
However, when fans tuned into season two, some of them spotted a noticeable change in Number One’s physique.
source: express.co.uk