An improvement on the previous sequel, though that isn't saying much - fans won't be disappointed, but it could have been a lot better.
Sequels, and particularly sequels to comedies, are usually defined by the Law of Diminishing Returns. However, the first sequel to Austin Powers was so abominably disappointing that it has left room for the 'threequel' to actually be an improvement. It's true that it shamelessly rehashes all the best (and several of the worst) jokes from the first two films, but it remains watchable, largely thanks to the presence of Michael Caine as 'Nigel Powers'.
The most important thing to say about Austin Powers 3 is that if you intend to see it, you should see it as close to its opening weekend as possible, because the best gags (including the hilarious opening sequence) are likely to be spoiled for you otherwise by Unscrupulous Reviewers. And if you see them coming, they won't be nearly as funny.
Preparation H
The plot is as flimsy as you might expect and doesn't bear close scrutiny. Basically, Doctor Evil (Myers) and Mini-Me (Verne Troyer) escape from prison and formulate another plan to achieve World Domination. The plan involves a lot of time-travel, "tractor beams" and the acquisition of "Preparation H" (Preparations A through G were unworkable) from another Evil Genius, Goldmember (also played by Myers).
For some reason, the plan also involves the kidnapping of Austin's father, Nigel Powers (complete with hilariously dodgy teeth and glasses), so Austin travels back in time to rescue his super-agent Dad and hooks up with an old girlfriend (an afro-wigged Beyoncé Knowles as 'Foxy Cleopatra') into the bargain.
To be fair, some of the gags are very good indeed. However, a lot of Myers' humour involves repetition and deliberately dragging out a punchline, and, inevitably, sometimes this works and sometimes it doesn't. The other problem is that some of Myers' characters (Fat Bastard, Goldmember) aren't as funny as he thinks they are.
A Touch Of Class
The acting is pretty good - Beyoncé Knowles doesn't have much to do except look foxy and deliver a catchphrase or two ("You're under arrest, sugar!"), but she does it very well. (In fact, she should be given her own TV show immediately.) Similarly, Michael Caine adds a touch of class and seems to be enjoying himself immensely. The 'subtitled cockney' scene is a definite highlight.
As for Myers, he's much funnier as Dr Evil and Austin than he is as his other characters. Luckily the other support characters (plus Fred Savage as 'The Mole') are all present and correct to keep the gags coming - the Scott Evil (Seth Green) subplot is particularly good.
In short, then, if you enjoyed the first two films then the chances are you'll enjoy this, although for every gag that works, there are a large handful that don't. To which the natural response is, of course, "What do you expect? It's an AUSTIN POWERS movie..." Worth watching, but not as good as it could have been.
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