Alex Mack (Alexandra Louise Mack) is a teenage girl, who simply wants to live her life like any other teenager. Having gained superpowers (to move objects telekinetically, shoot electric zaps through her fingertips, morph into a puddle of liquid, and glow when she gets nervous) during a chemical spill accident on her first day of junior high school, Alex now must contend with both the drama of the average teenage life and the dangers of her not-so-average powers.
Alex Mack used to be your average teenage middle school girl, living with her parents and older sister in the corporate town of Paradise Valley. One fateful day after her first day of junior high school, a wayward truck containing chemicals had crashed into a fire hydrant and doused her with a strange liquid . It's not long after when Alex realizes the chemical has done something to her. She now has the power of telekinesis, can shoot electricity through her fingertips, and morph into a silver puddle. She also turns golden yellow when experiencing extreme emotions, a source of much anxiety. She confides only in her sister Annie and her best friend Raymond about her powers. Now not only does she have school to deal with, but keeping her secret from everyone else, including her parents, for fear of what the chemical plant CEO, Danielle Atron, an organization out to find the 'GC-101 Kid' to bring her in for horrifying experiments.
Alex appears a teenage Caucasian female with long blond hair and blue eyes, typical of superhero appearance. Unlike most others, her apparel is subject to change, although her choice of clothing is more tomboyish. She also has an outlandishly odd habit of wearing hats (!?WTF!?), regardless of whether or not she is indoors.
Alex contains the typical traits of a teenage superheroine, being somewhat meek but a good natured person at heart. She matures over the series dealing with many affairs such as a love interest or the complication of her powers. However, she has often at times daydreamed of using her powers on those who have wronged her, but shows restraint knowing she can't risk anything.
After having been doused in the GC-161 mutagen, Alex gains a rather strange assortment of powers that mature with her as she grows up. She possesses the ability to fire torrents of yellow energy from each of her fingertips either as an offensive measure or to disrupt electronics. She also possesses telekinesis, moving objects by focusing her mind on them. Most infamous of all is her ability to morph into a liquid residue that almost always collapses into a puddle, allowing her to sneak about unnoticed, despite the sloshing sounds she makes. Not only does it give her a wider range of sight, but she retains the ability to talk, although her speech is slightly garbled, as if put through a water filter. She also retains all sense of feeling, still exhibiting feelings of pain when stepped or stomped on or ticklish when her body is twirled about.
Alex's powers evolve, her lightning powers now more focused into beams of energy and her liquid form changed into a silvery reflective surface instead of the jelly-like appearance from before. She also gains the ability to morph and absorb other people into her mass by mere touch, ideal for escorting multiple people out of danger. She also gains superstrength by eating curry and even blows bubbles at times. A passive ability has her face glowing gold whenever she is embarrassed.
GC-161 is a chemical compound of golden color that was being developed as a weight-loss miracle drug by chemical plant owner Danielle Atron. When taken in large doses, it has considerably dangerous side effects, including an ability to glow, telekinesis, electromagnetic manipulation, and the ability to metamorphose into a puddle of liquid. Many of Alex's problems seen throughout the series are a result of her powers, usually caused through malfunctions or lack of control.
Danielle Atron desires to capture the GC-161 kid (Alex) and study its effects on "him" (she and many others assume it was a boy who got soaked in the chemical). More importantly, Danielle Atron fears that if the media learned the truth of GC-161 through this kid, she would lose the billions of dollars GC-161 would stand to make and her reputation would be ruined. Other test subjects, like a chimpanzee, several rabbits, and an unwitting test family, also developed lesser forms of Alex's abilities via small doses of GC-161.
Alex later meets with one of the original researchers of GC-161, who reveals that the chemical was first developed in the late seventies, and that he knew of the danger it possessed (having tested it upon himself and gaining temporary powers). Though he tried to warn his superiors of the dangers of GC-161, Danielle Atron refused to listen, and so he fled to the woods to live as a hermit.
Louis eventually accidentally learns of Alex's powers, but agrees to keep them a secret. However, when he blabs about them to Ray while on the phone, he is captured by Plant security and tortured into revealing Alex's identity. After having Alex and her parents imprisoned and with GC-161 ready to be sold within hours, Danielle Atron arranges to destroy her chemical plant with explosives, thus eliminating all evidence of her sinister activities. Fortunately, Alex's friends arrive to rescue the Macks, while Dave reveals the truth about GC-161 and Alex (having eventually learned her identity) to the authorities, who see to it that Danielle is arrested in time.
Afterwards, Alex's father George develops a special antidote that can restore Alex's mutated state to normal. It is unknown what Alex chooses to do with the antidote afterward.