By now, most us are familiar with the basic idea behind Assassin's Creed: a historical action-adventure game set in an open world that you explore while battling the Knights Templar. But Black Flag, which is mostly set in 1715, tweaks the formula a little.

You take on the role of power-obsessed pirate captain Edward Kenway as he sails the Caribbean; in the modern day, you are using the animus at an Abstergo subsidiary to gather memories that the company plans to use for entertainment.

Stealth is still rewarded; rushing headlong into the fray is likely to get you killed. And you must still open up areas for exploration in cities by climbing the high points.

But it feels like Black Flag opens up a lot more quickly. It's probably a necessity, given that the game is set on the high seas, with trips to Havana and Nassau to break things up.

I was a bit sceptical, being a longtime Assassin's Creed fan. But the new elements and challenges derived from the sea battles are what makes this game worth playing. It's hard to keep a series like this feeling fresh, but the addition of the sea travel just about does it.

Once you've visited an island, you quickly get back there, which cuts out the feeling that you're pointlessly sailing. The ship, the Jackdaw, can be upgraded, which adds an additional impetus to keep playing, plus your crew can be recruited, as in Brotherhood. As before, there are plenty of side missions to carry out, if you don't feel the urge to rush through the story.

Visually, Black Flag is on a par with the previous titles, but the next-gen consoles will be head and shoulders above it in terms of environment textures and detail.

Visual tweaks aside, the question being asked how long Ubisoft can keep going with Assassin's Creed without it becoming tired; for now, it's clear sailing ahead.