She Spies > Season 1 > Episode 17 - The Replacement
Even though Natasha Henstridge isn't in this episode for long, it's still good.
In the begining the spies are after a thief known as Indigo. The lights go out, and fight begins, the lights come on and there is no Cassie.
So the plan is to get Cassie back before the guy steals the royal jewels on display. Luckily they get some help from Nick Shields (Brooke Burns). She is a super tough, pretty hot looking agent who has been after Indigo for some time.
They eventually figure out that Indigo is really a fashion designer. Naturally Shane, DD and Nick go undercover as models to find Cassie.
Needless to say they get Cassie back, and help Nick Shields from going to the dark side.