At the end of Halo 3 the Covenant, Halo's long-running alien bad guys, were thought disbanded and destroyed, with the Elites not exactly on the side of the human race, but not exactly against them.
So why, four years after the events of Halo 3, are they on Requiem, the Forerunner planet on which Halo 4 is set, fighting the Prometheans, the Forerunner defence force?
During a Q&A following a live gameplay demo at Eurogamer Expo this afternoon, Halo chief Frank O'Connor touched on the Covenant's mysterious involvement in the game.
"You definitely saw the Covenant fighting the Prometheans there," he said. "The Prometheans were objecting to their presence on Requiem, which is the world the story takes place on. If you think about human society and human civilisation, we're hardly monolithic. There are lots of different cultures and factions. You're going to find out about this particular faction and their particularly virulent form of the former Covenant with some really enthusiastic shall we say religious beliefs about the Forerunners.
"And they never gave up hope. You can only imagine their delight and surprise when they find out at least some of their myths and some of their legends about the Forerunners turn out to be true. That's something they discover on Requiem initially in a surprising way, but the game will have a few twists and turns, so you'll definitely find out more about that both in the campaign and in Spartan Ops. That's a big part of the Spartan Ops story."
Another fan asked, will Master Chief have a jetpacks?
"I can confirm there will be jet packs in multiplayer," O'Connor replied. "I'll tell you that now. It has been nerfed a little bit since Reach. It doesn't work in exactly the same way. You can't, for example, hover at the apex of your jump and rain grenades and sadness. But the jetpack is still in the game, and I'm not going to talk about the campaign because we already got close to spoiling the story for you there."
One fan asked about ranked multiplayer, Halo 4's networking and whether there will be dedicated servers. O'Connor confirmed Halo 4 is a peer to peer game - as expected - but said improvements have been made over past Halo games.
"Networking is peer to peer," he said. "It's obviously an Xbox Live staple and there are really good reasons for it. I realise PC gamers like having dedicated servers and there are some other games that have dedis. Peer to peer gives us a broader reach into the existing ecosystem. We made some tuning to the network performance. Hopefully you guys will like the improvements.
"We'll have more to talk about skill-based ranking closer to launch. There is an experience system connected to both Spartan Ops and competitive multiplayer."
Halo's trademark four-player campaign returns, but this time all four players assume the role of the star man himself: Master Chief.
"I can confirm we have four-player co-op campaign and of course Spartan Ops [Halo's episodic co-op story mode] supports up to four players. It also supports matchmaking, so if you're behind in the series you can find people to play through it with you. That's pretty much the ideal experience because then you can play on higher difficulty levels."
He added: "When we did Halo 3 you had Master Chief and three Elites and a back story that didn't have any severe impact on the game other than the Arbiter. The decision was made very early that that creates too many problems for a story that's supposed to be about Master Chief this time. The reality is everyone's a Master Chief, so your big brother doesn't win that argument every time."
And finally, a fan asked if Recon armour was unlockable in Halo 4. Frank's reply: "Email me and I'll unlock it for you in Halo 3.
A video of the Halo 4 Eurogamer Expo developer session is below.