Our heroes' first adventure is called “The Secret of Swordfish”. This is not a fish, but a submarine capable of rising out of the water and flying at supersonic speeds. It was developed in particular by Philip Mortimer. Here is the cover of the first volume in the series.
https://www.bdparade.com/bande-dessinee-blake-et-mortimer-integrale-le-secret-de-l-espadon-1964

Difficulty ****