Hydra is the tale of St. George from the perspective of the Hydra he's made his mission to kill.
Musically, the song features some of Steve Lukather's best guitar playing and some exceptional keyboard work by both Steve Porcaro and David Paich.
The very next track, St. George & the Dragon is the continuation of the tale featuring some more clever keyboarding from Paich and Porcaro.
Sadly the rest of Toto's Hydra album is a bit uneven. There are some strong bits like 99 and
I think if Toto had chosen to write the entire album around the tale of St. George either as a concept album (a la Alan Parsons Project) or even a rock opera (a la The Who's Tommy) the end product would have been considerably stronger.
In 1984 Toto composed and performed much of the music to the Dino DeLaurentis version of Frank Herbert's Dune and some of the material from that, is musically similar to the material on Hydra. Desert Theme could just as easily have been an overture to open the tale of St. George, Take My Hand would have fit well as a stirring emotional interlude in the tale.