When you reach Captain Keyes, use the switch to
let him out of the covenant cells. When you've done that, quickly run to
towards him and try and throw a plasma grenade at him.......... but there
will an invisble barrier still there. This will make the grenade stick to
you but the game should go to the cutscene and you will die whilst the
cutscene is playing. When it goes back to the game you will be dead and
you'll recieve a checkpoint. All you can do is listen to the marines
fighting. You'll be stuck there until you restart the whole level.
At the end of the level, as you press the switch to turn off the shield
barriers, throw a plasma grenade on Keyes and the level should end but he
will be dead. Sometimes this will cause the game to go straight to the next
level.