At one point in the book, Plutarch Heavensbee claims that he didn't know that Katniss was going back into the Games. This would seem very odd if true, especially given that he was the Head Gamemaker and was already involved in the conspiracy against the Capitol. Was he lying? If so, why? If not, what might the original plan have been given that Katniss wouldn't actually be in the arena for them to retrieve?
He didn't know about it way back during the Victory Tour, only later on.
The quote you're thinking of is from the final chapter of Catching Fire:
“Neither you nor Peeta were told. We couldn't risk it,” says Plutarch. “I was even worried you might mention my indiscretion with the watch during the Games.” He pulls out his pocket watch and runs his thumb across the crystal, lighting up the mockingjay. “Of course, when I showed you this, I was merely tipping you off about the arena. As a mentor. I thought it might be a first step toward gaining your trust. I never dreamed you'd be a tribute again.”
And the event he's referring to is when he danced with Katniss during the Capitol stage of her and Peeta's Victory Tour, months before the Quarter Quell:
“Are you planning the Quarter Quell Games already?” I say.
“Oh, yes. Well, they've been in the works for years, of course. Arenas aren't built in a day. But the, shall we say, flavor of the Games is being determined now. Believe it or not, I've got a strategy meeting tonight,” he says.
Plutarch steps back and pulls out a gold watch on a chain from a vest pocket. He flips open the lid, sees the time, and frowns. “I'll have to be going soon.” He turns the watch so I can see the face. “It starts at midnight.”
“That seems late for—” I say, but then something distracts me. Plutarch has run his thumb across the crystal face of the watch and for just a moment an image appears, glowing as if lit by candlelight. It's another mockingjay. Exactly like the pin on my dress. Only this one disappears. He snaps the watch closed.
“That's very pretty,” I say.
“Oh, it's more than pretty. It's one of a kind,” he says. “If anyone asks about me, say I've gone home to bed. The meetings are supposed to be kept secret. But I thought it'd be safe to tell you.”
At this point, the arena was already being made - which makes sense, because Hunger Games arenas must take months or even years to build and set up before the Games begin. But the exact nature of the Quell wasn't yet known to Plutarch, the new Head Gamemaker. Most probably it wasn't known to anyone yet; the Victory Tour was still underway, Snow had only just decided that Katniss and Peeta's love story wasn't convincing enough, and so the idea of sending them back into the arena probably wasn't on the table yet. (This is assuming, as seems reasonable, that this idea was created specifically in response to Katniss and the revolutionary movement she inspired, rather than having been planned for the Third Quarter Quell ever since the Hunger Games started.)
Obviously, as Head Gamemaker, he would have known about this plan well before it was publicly announced. Perhaps he was even the one who suggested it to Snow, as in the films. But it's not particularly strange that he didn't know about it way back during the Victory Tour.