Imagine you've just watched Return of the Jedi. You've seen the end of a good film, the finale of a great trilogy and, whatever you may think of its denouement, it is over.

Now imagine George Lucas popping up in your living room five months later brandishing a VCR. He's got a deleted scene you simply must see. Mass Effect 3: Leviathan feels a bit like that deleted scene.

But, all credit to BioWare, Leviathan offers a generous amount of content to coax you back. At the centre of it all is the mystery surrounding the Reapers' origins and the tease of something that was once even more powerful. In the Mass Effect universe, these are true revelations - revelations that BioWare chose to omit from the main game. Like the day-one DLC From Ashes (and, arguably, the Extended Cut), these story threads are so tightly woven into the narrative that it's almost frustrating they were ever separated at all.

What Leviathan offers will likely excite and infuriate in equal measure. There's a glimpse at something hugely important to the game's lore - and maybe even the series' future - but BioWare's surprises feel hamstrung by having to work within the framework of the existing game.

Shepard's underwater exploits have been teased heavily during the DLC's promotion. But you'll be down there mere minutes.

Unlike Mass Effect 2's mostly-excellent DLC, which could freely build on the aftermath of that game's events, Leviathan can only be a pit stop on the Mass Effect 3 roadmap. The goalposts have already been set for the end of the trilogy, and Leviathan (which could really fit anywhere within the latter half of the game) is forced to operate within that.

It's worth noting that those with Leviathan installed do get a slightly tweaked finale - but it is a fleeting moment of exposition. Such is the information learnt about the Reapers, it would be odd if Leviathan's events were not mentioned somehow.

Happily, Leviathan's ability to delve back into the pre-finale world of Shepard can also be something of a bonus. A lot - perhaps too much - has been made of Mass Effect 3's endings, so much so that it is now impossible to talk about the game without touching on the subject. But that conversation usually obscures the vast majority of a fantastic adventure, something which Leviathan's dazzling planet-hopping and tense combat sections revel in bringing back to the fore.

In short, Leviathan's gameplay reminds you why the series is so good and of the potential it still has. The adventure spans three major missions, each interspersed with visits to a new area of the Citadel. And it's here, in a mysterious clue-filled lab, that BioWare is able to really have some fun.

Within minutes, the traditional pattern of "meet quest giver, run off to mission" is abruptly altered. Shepard is wrapped up in a detective operation, scouring the environment L.A. Noire-style for pointers as to what to do next. There are fan-pleasing references and nods to old Mass Effect plots aplenty, as well as lots of time spent with EDI, one of the surprise highlights of Mass Effect 3.

"In short, Leviathan's gameplay reminds you why the series is so good and of the potential it still has."

It's worth mentioning the difference felt by having all of the main voice cast back on board. Mass Effect 2 DLC had to skirt around the fact that none of your squadmates had any new dialogue. Leviathan has character moments in abundance and mid-mission nattering from all of your team (leaked script details suggest voice recording took place in parallel with the Extended Cut, which makes sense). There's even some more Normandy-based chatter.

The story is not as well rounded as the character-led episode Lair of the Shadow Broker, but Leviathan's new faces do an admirable job of keeping the sci-fi intrigue believable. Leviathan's tale weaves the utterly fantastical - even for the Mass Effect universe - with a regular dosage of human drama. Once again, BioWare succeeds by wrapping complicated sci-fi around the dynamics of a family struck by tragedy, in a similar fashion to the oft-overlooked Overlord DLC for Mass Effect 2.

The locations are also real stars of this add-on, however briefly they are visited. Corridor sequences are wisely kept to a minimum as the game's trio of planetary excursions take you on a whistle-stop tour around the Galaxy Map. There's a Dead Space-style creepy mining colony with a chilling atmosphere and some truly unnerving moments. Then there's a precarious, rusted settlement left clinging to a giant cliff face, mid-Reaper attack. And there's a gorgeous ocean world, whose undulating waters rock the creaking floor beneath you.

Price and Availability Platforms: PC, PS3, Xbox 360

Price: £7.99/800 Microsoft Points

Availability: Out now on PC and Xbox Marketplace worldwide, and PS3 in US. European PS3 release due later today.

You'll need to access your private message terminal on the Normandy to start the Leviathan plotline.

Gameplay innovations are not limited to the CSI-style investigations in the Citadel lab. Leviathan takes players underwater for the first time in a brief section of aquatic exploration whose looks are reminiscent of BioShock 2's sojourns outside Rapture. Less impressive are the combat objectives borrowed from Mass Effect 3's recent multiplayer expansions. One section sees Shepard having to escort a drone around the battlefield (introduced in the Earth multiplayer DLC). Another sees you lugging packages from one area to another (introduced in Rebellion and referred to by players as the "pizza delivery" round).

It's hard to complain, though, when the pack contains such a depth of content, such as the selection of new weaponry and gun mods. There are even bonus planet systems to explore for those who like to spend hours trundling around the Galaxy Map reading planet information and scanning for War Assets.

There are about a dozen new Assets to collect, although their value is somewhat moot since the Extended Cut lowered numerical requirements for the best ending. Still, if you want to discover the exploits of a team of vorcha fighter pilots, or find out why a batarian leader is enraging his peers, BioWare has you covered.

Leviathan is a rich adventure for those who are willing to reopen the door on Commander Shepard's story, and a worthwhile chapter of lore within the Mass Effect canon. But for those who have already moved on it is perhaps reassuring that, at the end of it all, those goalposts lie largely unmoved.