From the official website:
Storyline
Jagged Alliance: Back in Action takes you to a fictitious country called Arulco: a ruthless dictator has seized absolute power and only a small group of rebels are still able to resist the tyranny. You command the rebels and the best mercenaries in the world are fighting on your side, provided you keep your troops supplied with plenty of cash and morale! You will require diplomatic skill and nerves of steel to win against the reign of terror.
Gameplay
Unlike the original game, Back in Action uses a combat system called "Plan & Go", which combines RTS gameplay with turn-based elements.
"The biggest improvement (outside of the new 3D engine and modern graphics) is that the game can either be played in a new RTS style mode, or the turn based system from the first game, or a hybrid of the two. The hybrid mode shifts from real time to turn based on criteria specified by the player. For example you can play in real time mode until you discover a mine or come across a bad guy. From there you can then switch into the game’s “Plan and Go” combat system. This mode is that you can key actions of your team together which brings out the real strength of the system.
{C For example, let’s say you want to have one guys sneak up behind a bad guy and cut his throat but then have a sniper ready to take him out if he fails. Well you setup the actions of the first guy along a timeline along the bottom and then when you have the kill action taking place you can have the sniper take the shot at exactly the same point in the timeline. If you’ve ever programmed Flash it’s very similar to the Flash timeline in execution and it’s something that gamers should be able to pick up pretty quickly. You can also do fun things like having your squad move into position and then open fire simultaneously or whatever you heart desires.
The laptop interface from the original is back but not all of the weapons from the original will be back due to licensing issues (though there will be replacements that are nearly identical). The game will get all new voice over work from the stars of the original which ensures that one of the highlights from the first game comes back from the second."[3]