Dwayne Johnson aka The Rock cannot act, but that doesn't stop The Scorpion King from being an enjoyable film.
In a plot that doesn't jibe at all with what's depicted in The Mummy film series, Mathayus (Johnson) is recruited to assassinate Cassandra, the sorcerer (Kelly Hu) for Memnon (Steven Brand), the king of Gomorrah. Memnon seeks to fulfil the Scorpion King prophecy as being the ruler who will unite all the warring Middle Eastern and Northern African factions, aided by Cassandra's clairvoyance. But Memnon is cruel and as you might've guessed, not the one destined to fulfil the prophecy.
The action and fight sequences are what make the film work to some degree (and Kelly Hu's definitely easy on the eyes, which doesn't hurt either). Besides that, the film is good to laugh at, for some of the dialogue (which is some hybrid ancient/modern dialect of English) and the seriousness with which the actors take themselves. It seems that Johnson does a better job of acting in the wrestling shows on TV (or even the episode of Star Trek: Voyager that he appeared in).
The Scorpion King is a good time-killer and is definitely worth a rental. Be prepared to check your brain out for a while and don't expect any long-term effects.