The phrase in the book is:
So they think you're dead, Andrews, and they have now played their last card. So we must hold ours back. You're going to remain dead—for a little while longer.
They're using an analogy to a card game such as poker. They have put their last card down, or made their intentions clear.
But they're going to keep their last card a little longer, and keep their secret from the others: that Andrews is not really dead.
What you should realize about the Austin Powers movies that may not be immediately obvious to a non-native speaker is that the character is written by USA natives to be an exaggerated parody of a British guy from the 60's. As such, a lot of his tag lines aren't so much phrases in common use in England, but rather an American's view of phrases in common use in England in the 60's.
The general concept behind the character is that he still has the mentality of a 60's Alpha male from a James-Bond knockoff movie. In his mind the "sexual revolution" has barely begun, women exist purely for his personal gratification, and are probably just waiting for a positive signal of interest in his part to jump his bones. Any attempt to talk to him as an equal, as a modern woman would, he misconstrues as sexual interest.
So you have to keep in mind when watching those movies that in nearly every scene this is a guy who is going to be inappropriately sexual in a (to an American) cartoonishly British 60's way.
Looking at the "Oh behave" in that light, what you should see is a guy who thinks the person he's talking to is making sexual advances, and is basically saying, "Great idea for later, but we have something else we have to take care of here first."
Best Answer
There is no idiomatic expression here overall, except for the verb: to be back to something. That said, in regular speech, to describe a situation, a speaker will often say: Now, we're back to [some condition or state]. And the "to be back to" is followed by a condition or state can be said by a person or it can be attributed to a situation. Here, it's for a situation, "Now, it's [the situation] is back to A [condition].
Of course, to be back to something just means to have returned to a place, situation or condition. A paraphrase of the sentence would read: We have now returned to a situation of brightness. And brightness, well, that is a lot of light, or perhaps better said, not darkness....