The "I woke up and suddenly I was a different version of me" scenario has been done--probably best in Big with Tom Hanks--but after the magical transformation in that film , everything that proceeded from there was pretty much reality based. In 17 Again, Mike O'Donnell (Zac Efron) is a high school basketball standout who gives up a promising sports career to be with his pregnant girlfriend and raise a family. Fast forward twenty years and the adult Mike (Matthew Perry) is on the outs with wife Scarlett, (Leslie Mann) and his communication with his now high school aged son and daughter leaves a lot to be desired.
Mike's "spirit guide" (a crusty janitor at his old high school) sees that Mike longs to change his circumstances and propels him back into his 17 year old body. Mike appears at the house of his old geeky/trekkie friend Ned (Thomas Lennon) who doesn't recognize him in his teenage body (even though Mike looks the same as he did when he really WAS 17) and immediately tries to KILL the young Mike. Most people would call 911 if they encountered an intruder, but Ned swings battle axe and assorted weaponry/memorabilia he has around the house in a wild drawn-out scene in which Mike is fighting for his life while trying to convice Ned of his true identity. This is where you say to yourself: This is totally cartoonish--and you feel a little disappointed (if you LIKE to have things portrayed in a reality based manner) and must then decide to just go along for the ride and laugh and enjoy whatever else follows. And what follows ARE a lot of laughs. (The teenage girls in the theatre were giggling and snickering and spitting up their soft drinks all the way through.)
Once Ned is convinced of Mike's identity, he decides to act as his old/young friend's father and enroll him back into school. Ned is smitten by the high school principal, Jane Masterson, (Melora Hardin) and that's the beginning of a cute subplot where Ned stalks Jane until she consents to go out with him. They eventually discover that they are both fantasy nerds and made for each other.
Meanwhile, the young Mike gets back on the basketball squad and discovers that the school bully, Stan, (Hunter Parrish) who also happens to be Mike's daughter's boyfriend, has been picking on his son. (You'd think that bully Stan would steer clear of terrorizing his girlfriend's brother just to stay in her good graces, but that's too much reality-based logic for this film.) Mike feels compelled to defend his son and discourage the relationship between his daughter and Stan, which adds to the comedic fun of this movie because nobody knows why he should care. There's more fun when teenage Mike (who has been renamed "Mark" by the way) interacts with the adult Mike's wife, Scarlett, who possesses enough visual acuity to pinch his cheeks and tell him how much he reminds her of her husband--who she is divorcing. Then, when his daughter develops a crush on him and wants to get cuddly, Mike does a squirming act that Houdini himself would be proud of! The young Mike's challenge is to get his kids back on track and somehow turn his wife's head around to where she'll change her mind about the breakup.
17 Again is a romp--just don't try to make much logical sense out of it.