It is likely to be one of the biggest entertainment releases of the year, if not the decade, and may well become a major obsession for the gamer or sci-fi fan in your life – but what exactly is Destiny?

The epic space action game is a hugely ambitious online shooter, which mixes several popular video game genres into one vast adventure. But even experienced video game players do not know exactly what to expect.

Although there have been planet-loads of multiplayer online games in the past, few have sought to make it so easy and intuitive for people to enter, explore the story and meet up with others to take part in missions. Destiny wants to change the way we think about online gaming – whether it succeeds or not will be down to the capability of its server network and whether there is really enough in there to keep gamers coming back. Early indications have been positive – a limited beta test of the game which released for two weeks in August was played by 4.6m people.

If you know someone who’s excitedly waiting for a copy to turn up on Tuesday morning, here’s what they’ll be doing – and why they may be doing it for a quite a while...

What exactly is Destiny?

Destiny is “shared world shooter” designed by Bungie, the development studio behind the classic Halo series of first-person shooters on the Xbox. The game combines elements of the first-person shooter (FPS) and role-playing game (RPG) genres into one gargantuan experience. Players explore the galaxy, battling aliens and unlocking new weapons and abilities; all the action takes place in a single online universe so participants are able to meet each other and team up to defeat enemies. It is going to be a major commitment, so don’t make any plans.

What is the story behind the game?

Destiny is an epic science fiction story inspired by those chunky “space opera” novels by authors like Peter Hamilton and Alastair Reynolds. The action takes place 700 years in the future. A vast alien being known as “The Traveler”, which looks like a small moon, has arrived at Earth and aided humanity into an accelerated period of technological advance. Using interstellar colonisation ships, human settlements have spread out across the galaxy, leading to a golden era of discover, expansion and reasonable property prices.

However, an evil force that has been tracking The Traveler across the cosmos has finally tracked it down, all but destroying mankind’s protector. Now, various alien armies are raiding the solar system and only one Earth city is still protected by our moon-like extraterrestrial saviour. This is The Tower, a vast skyscraper, which acts as the central social hub of Destiny.

So what do you actually do in Destiny?

Players take on the role of guardians, who are effectively heraldic knights of the future. They must work together to defeat the raiding alien races and save humanity. When the game starts, each participant gets to choose from three different character classes: hunters, warlocks and titans. Each has its own special skills and abilities. Through the game, as players kill enemies and complete missions, they can discover new weapons, items and abilities, further personalising the skills of their onscreen avatar.

How long does the game take to finish?

Honestly, no one knows. There is a central story to experience and complete, but whether this finishes at the end of this first title, or whether it continues into subsequent titles is unclear. However, like other role-playing games, Destiny does not have a traditional end point. Even if the story is finished, there will still be missions to take on and complete with friends.

Also, throughout the game, each victory helps to “level up” your character and open new abilities. Leveling up in this way may be more important to some players than simply rushing through the narrative as quickly as possible. The larger “raid” missions are also likely to expand the gameplay time – they are more complex and challenging and may take up to two hours to complete.

Furthermore, as with other online games like World of Warcraft, it will be possible for Bungie to keep adding new content. See “how much will it cost” below.

Certainly, it took around 4-6 hours to see all the key content in the beta test that Bungie released to players in August, allowing them to try out the game – but this was, apparently, a tiny fraction of the missions and story content available.

How is the game structured?

Every player starts the game with a basic spaceship and a mission map; the map shows the various missions available to them throughout the solar system. There are several different types of activity. Story missions guide the player through the game’s narrative, in the epic quest to save the Earth; strike missions are quick 20-minute fights in interesting locations; patrol missions are open-world exploration areas which let you whiz about on speeder bikes; and raids are more in-depth quests that require you to team up with five other players.

How each player takes on the game is up to them. It’s possible just to do the story missions and treat the game as a single-player role-playing adventure. However, most of the rest of the missions require players to team up with two others to create “fire teams”, who take on missions together. You can make “fire teams” with your friends or you can meet people within the game and team up. There is no player-on-player shooting in these missions, it is entirely co-operative.

However, players who DO want to shoot each other can also use their map to find “crucibe areas”. These are multiplayer shoot-outs that work like the online modes of games like Call of Duty – ie you are shooting other players online, either in teams or every guardian for themselves.

Where do the missions take place?

Bungie has not revealed all the areas that will be in the game but we know that players will be able to visit Earth, the Moon, Venus and Mars. The moons of Jupiter and Saturn have also been mentioned. On Earth, we have already seen one of the main locations, a wrecked Soviet cosmodrome, which has become an alien stronghold. Some of the players who took part in the game’s beta test this August also got to visit the Moon.

What makes Destiny different?

Although the game contains lots of familiar elements, it is the way it combines them that marks it out. You get to shoot lots of aliens, just like in Halo, but you also develop and customise your character, carefully personalising its appearance and special abilities – just like in an RPG. And as in a massively multiplayer online game like World of Warcraft, you get to meet other players.

For example, you may be taking on a strike mission with two friends, when you suddenly, bump into another group of players on their own quest. You can either ignore them, or help them out and share the spoils. Often there are special “public encounters”, where a larger enemy suddenly turns up, requiring several groups of Fire Teams to dispatch it. You do nt have to accept these missions but they often provide rare and advanced weapons and equipment as rewards.

Bungie’s big idea with Destiny is to subtly encourage players who do not usually go online to work together to defeat the game.

How much will it cost?

The game retails at between £45-£55 depending on where you buy it – check out specialist and online retailers for special deals; supermarkets are also likely to provide heavy discounts.

If you are playing on Xbox One, Xbox 360 or PlayStation 4 you’ll also need to subscribe to the comparative online gaming services to play (PlayStation Plus on PS4, Xbox Live Gold on Xbox). This is around £35 a year. PlayStation 3 allows free online gaming.

If you start the game on the Xbox 360 or PS3 but then later upgrade to the Xbox One or PS4, Bungie will allow you to download a new version of the game for free – as long as you stay with the same brand (ie as long as you upgrade from Xbox 360 to Xbox One or PS3 to PS4). All your progress will be transferred to the new machine. This offer expires on 15 January 2015.

Bungie will also provide a series of download packs for Destiny – it already has one planned for December. These will add new missions and planets. The series has been designed to run for ten years, so it’s likely we’ll also see at least two sequels.

I have an Xbox One, can I play with my friends on the PlayStation 4 or Xbox 360?

No. Although Destiny effectively lives on one computer network, you can only meet and play with people who own the same console as you.

Is Destiny suitable for my child?

Destiny has been given an age rating of 16+ by Pegi, the certification board that serves Europe. The game contains violent encounters, mostly with alien monsters. It is unlikely to be as gory as some other first-person shooters but you should treat the age rating as you would a film rating from the BBFC.

The game will also require significant time investment from players, so if you buy it for a teenager, it may be a good idea to negotiate with them in advance about the times when they will be playing.

The game also allows communication between players via console gaming headsets. However, your children will only be able to chat with people they actively invite into their fire teams, or who invite them into a fire team of their own. Communication between complete strangers in the game is only available through a series of pre-programmed animations, such as waves and dance moves.

It is worth watching or playing the opening sequences of the game, even if you are not interested in taking part yourself – this will give you an idea of the content and what it is all about.

Is Destiny actually any good?

The beta test went down well, although some fans are still wondering if there will be enough content. The environments shown so far have not been massive when compared with, say, the area available to explore in Grand Theft Auto V – but then we have only seen a few missions. “One thing we’ve struggled to communicate is the sheer scale of the game,” said Jonty Barnes, head of production, when the Guardian spoke to him at Gamescom.

The studio is, however, confident there will be plenty to do – the questions are whether there will be enough variety, will it be easy to find people to play with and will the servers hold up? The launches of other major online games, including Battlefield 4, GTA Online and SimCity, were plagued with technical difficulties, often for several weeks. Bungie says it has a vast and stable infrastructure, which it has intricately tested. We will soon see.

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