This plot description should give you an idea what it's all about.
Not too many spoilers I hope.
Sydney (Tia Mowry) is a perfectly normal seventeen year old student who recently moved into town with her family: Her younger brother Willie (Tahj Mowry) who is a science kid and working on an age control project, her Dad Barry (Phillip Jarret) and his new wife Monique (Tonya Lee Williams).
Sydney basically hates her new life. She's having a hard time fitting in and desperately misses the big city (Los Angeles).
When Sydney is not at school she works at the local supermarket. Her only real friend Julie also works there. Whilst stocking groceries the girls chat about boys mostly - especially Todd (Daryn Jones) who Sydney likes a lot.
The story unfolds as Willie's latest science project goes wrong. Smoke producing chemicals from a youth experiment forces him to abandon the garage lab. He doesn't realize the chemicals spill into a box of soap.
That same day Sydney and Willie's grandparents are visiting. Only problem is that the elderly couple (played by Hope Clarke and Robert Hooks) can't stand the sight of each other and they have been divorced for many years. They spend every opportunity arguing over the smallest things.
Furthermore as Barry and Monique suddenly have to go visit some family in LA Sydney realizes she and Willie will be spending the next few days in the company of two bitter grandparents.
Things start to go really wrong when both grandparents accidentally use the soap that Willies chemicals spilled on. Grandma Cat (Tamera Mowry) wakes up finding herself looking 50 years younger than she did the night before.
Same thing happens for Grandpa Gene (Mark Taylor) as he drives back from the airport having dropped off Barry and Monique. He is quiet amused by his regained youth whereas Grandma Cat is rather disturb and decides to go look for Sydney.
Naturally Sydney is shocked to find her Grandma looking like she was her twin! It doesn't take Willie long to figure out what is going on and Sydney authorizes him to re-open his lab and find a cure to reverse the youth process. She wants everything back in place before Barry and Monique returns from LA.
It's almost like Sydney gained a sister. Being the same age she and Grandma Cat is able to bond in a new way.
The bitterness between Cat and Gene continues. When Ashley, the school queen bitch (Maia Campbell), asks Grandpa Gene out on a date, Grandma Cat gets furious and quickly accepts an offer from Todd who mistakes her for Sydney.
When Sydney finds out her grandmother is going out with her boyfriend-to-be jealousy kicks in.
They make an agreement however. Cat goes with Todd to the club called "The Orange Room" as planned pretending to be Sydney. Later they will make the switch.
Things doesn't go quite as planned though. When Sydney arrives at the club she finds Cat in a hot dance with Todd. Upset she storms out of the place.
Grandma Cat realizes her mistake and goes looking for Sydney followed by Grandpa Gene. The couple end up in a swing dance night club and before the night is over they re-discover the romance of their youth.
But time is running out fast. Willie discovers that Cat and Gene's situation is getting critical. He struggles to find a cure before it's too late!