Picking up the storyline following standalone TV movie Redemption, it’s four years since CTU was disbanded and Jack Bauer (Kiefer Sutherland) is back from Africa and in front of a Senate hearing into the illegal use of torture during his attempts to foil terrorist attacks. However, no sooner have the hearings started than FBI agent Renee Walker (Annie Wersching)  request an adjournment as the agency requires his input over a plot to infiltrate America’s computer infrastructure. Much to Jack’s surprise, he learns that one of those behind the plot is Tony Almeida (Carlos Bernard), his friend and fellow CTU agent whom he saw die in a medical facility.

It’s not giving too much away to say that while the past four years have involved the embittered Almeida in murky waters, he’s not about to let terrorists cripple his country and is, in fact, working undercover alongside former CTU agents Bill Buchanan (James Morrison) and Chloe O’Brian (Mary Lynn Raskub) on a mission to uncover deep seated conspirators within the government in the pay of Colonel Dubaku who is working to force President Taylor (Cherry Jones) to call off the invasion of Sangala, the African nation where his ruthless boss, General Juma, is committing genocide.

With the FBI compromised and no idea how high the conspiracy goes, Bauer, Almeida and Walker are forced to work alone to try and capture Dubaku and recover the device that has already caused a mid air collision killing 300. Initially repulsed by Jack’s methods,Walker gradually finds herself realising that sometimes extreme situations call for extreme measures.

 Weaving a complex web of betrayal and conspiracy that involves a variety of connected subplots, it keeps you guessing as to who may or may not be a traitor (though if you think one character looks decidedly weaselly, you’re totally right) and while it’s relatively less violent than past series, the body count and firefights remain high.

The new characters, who also include Jeffrey Nording as Walker’s boss and romantic interest Larry Moss and Janeane Garofalo as FBI agent Janis Gold, are solidly realised and, alongside Buchanan and O’Brian, there’s several other returning familiar faces from previous seasons, including one major surprise in the final hours.

Inevitably, Jack gets puts through the wringer, variously being beaten up, arrested, freed, tortured and being exposed to a lethal bioweapon while it’s never a good idea to rely on even recurring characters making it to the end of any particular episode.

Clearing up one crisis only to launch straight into another, with yet more double crosses, it’s high on nail biting tension and action - there’s even a terrorist attack inside the White House itself - while also maintaining a provocative debate on how far it’s permissable to go and what collateral damage is acceptable in defence of your country and freedom as well as examining the psychological and emotional cost of those decisions.

One of the best and most complex seasons yet (and the first released on both DVD and Blu-Ray), bonus features include deleted scenes, behind the scenes footage, commentaries and featurettes, including one UK exclusive. Roll on Day 8.