The incident took place at a condominium block in Hialeah, Florida, in the communal swimming pool. One girl innocently reached for a metal handrail while swimming - but thanks to an electric pump which was improperly grounded, the contact sent electricity rushing through the water.
What follows can be seen in the chilling CCTV video, which has been released by the local police department.
One man, who is sitting at the water's edge, immediately notices that something is wrong when the girl grips the metal bar and stiffens, her head thrown back. He reaches to take her arm and automatically recoils as the current zips through her body and gives him a shock.
Undeterred, he plunges his arms back into the water and breaks the current by grabbing the girl and heaving her up onto the poolside.
The other children scramble to safety - all except one girl, Daniella, who has been paralysed just short of the steps leading to safety.
She lies face-down and limp in the water for several seconds - forgotten in the pandemonium which has set in in around the pool - before her granddad, Freddie Cabrera, rushes to the rescue. As he pulls her out of the pool, the charge is so strong that it causes him to fall backwards.
Luckily, none of the young swimmers were seriously hurt, thanks in no small part to the reactions of the quick-thinking adult bystanders.
The two girls spent four days in hospital after the accident, along with Daniella's brother, Diego Cabrera, 10. Diego recalled the traumatic experience, describing the moment he realised his sister was hurt, saying: "She went unconscious... I was crying."
All three are expected to make a full recovery.