Following the death of his daughter, Fisher had nothing left to keep him from the most dangerous mission of his career. Sure, they've all been "the most dangerous mission" up to this point, but Splinter Cell Double Agent represents a radical departure from the series by shifting the focus from stealth to trust. It's Washington's trust you need to earn in order to escape this maximum security prison and infiltrate JBA.
Step one in helping Washington escape is simple: get out of your cell. Here the developers at Ubisoft steal right from "The Shawshank Redemption" and conveniently place a poster that conceals a hole in the wall to freedom. Washington has already started a riot in the prison, so guards don't take notice of your absence.
From there, you'll make you're way unseen through vents and above the cell block, prisoners and guards fighting below. Using your shiv, you break into a control room and duck into the shadows, a lone guard keeping an eye on the riot. Here's where your first choice comes in.
Do you kill the guard, slitting his throat with the shiv? The National Security Agency has granted Fisher plenty of leeway in his latest mission, but if Fisher goes too far off the grid the NSA will lose trust in its most prized agent. Fisher can still use non-lethal force to knock the guard out, either by a swift blow to the head or a chokehold from behind. But that risks triggering an alert, so the perfect move is to hide in the shadows and hack the computer in silence. Just don't let the guard see you or lethal force may be your only option.