The "Story Mode" can be played with another human player, or solo.
An advantage of the game is that you can build up a combo by killing mutants, shooting headshots, and more killing "stuff", but also, saving civilians, which is (finally) a good thing that you will do in this game (if you don't count killing mutants as a good thing). "If a player dies, points from his or her overall score can be quickly spent to continue playing"*, which is a con, because you will lose points of your total score.
At the end of each level, a player will receive a total score, which depends on how much time you died, or not dying, and accuracy on shots. In a level you can also pick up things, such as Grenades, Health-Packs and the so called "Slow Mo-Fo", which will put everything, temporary, into Slow Motion.
New Guns and Gun Upgrades can be bought from the cash earned after a level.

The Directors Cut and Mini games

After the completion of the Story Mode, you'll unlock the "Director's Cut", which unlocks a tougher Story mode, with harder-to beat enemies and a limited amount of continues.
You'll also unlock some Mini Games, such as:

• "Staying Alive" --- The one who survives killing mutants the longest time, wins.
  
• "Victim Support" --- You have to help a group of civilians escape.
  
• "Money Shot" --- A competitive Mini Game where four players try to reach the highest score by killing mutants.
  

I could write this, but I'm taking it off Wikipedia.
Agent G: Inexperienced but already highly trained and deadly, he graduated top of his class at the AMS academy and is now on his first assignment.
Detective Isaac Washington: A hard-drinking, ladies' man, Washington is also a habitual rule and heart-breaker. He took the assignment to exact revenge for his father's death. He has a habit of dropping F-bombs.
Varla Guns: The hottest stripper on the Bayou City club scene, Varla is also sister to crippled scientific genius Jasper Guns. She joins the agents to stop Papa Caesar, and a love triangle develops.
Papa Caesar: Deranged crime lord Caesar has forced Varla’s brother Jasper into devising a strange compound with mutagenic effects. With the help of Warden Darling he sets to work transforming the innocent inhabitants of Bayou City into mutants and monsters. He also likes Chinese food.
Warden Clement Darling: A strange and unsettling man who oversees a high security prison just outside of town, Darling is involved in gruesome scientific experiments to prolong the life of his elderly mother.

Starts in 1991, 7 years before the events of House of the Dead, AMS Special Agent G is given his first assignment and sent to a small town in Louisiana to investigate a series of disappearances and hunt down a deranged crime lord, Papa Caesar. Due to the presence of mutants, G is forced to team up with Detective Isaac Washington who is out to get revenge on Caesar for apparently killing his father.
In the first episode 'Papa's Palace of Pain', the pair first storm a mansion owned by Caesar out in the woods. Upon entering the basement, they find Caesar has fled, and the disabled scientist Jasper Guns, whom Caesar used, is horrifically mutated from injecting himself with an unknown substance. Upon defeating Jasper, G and Isaac meet Varla Guns, a stripper and Jasper’s sister who vows revenge on Caesar.
In the second episode 'Ballistic Trauma', G and Isaac track Caesar to a hospital that is swarming with mutants. After killing the mutants and a creature called the 'Screamer', the hospital is set to explode. G and Isaac escape in the nick of time with Varla on a motorbike.
In the third episode 'Carny', they stop at a mutant infested carnival to investigate the mutant situation, in the process defeating former circus entertainers turned horrific creature Nigel & Sebastien.
In the fourth episode 'Scream Train' Caesar escapes on a train with G and Isaac giving chase while Varla is left behind. The pair made their way through the mutant-infested train, defeating another mutant creature, the Crawler, in the process . The train then crashes in a swamp where Caesar drives off with Varla before throwing a tape to Isaac.
In the fifth episode, 'Fetid Waters', G and Isaac make their way to the swamps, finding an alternate way to wherever Caesar and Varla were heading for, and in the process, defeats another boss mutant called the Lobber.
In the sixth, penultimate episode 'Jailhouse Judgment', G and Isaac track Caesar to a high security prison. Before they enter, they run into the strange warden, Clement Darling, who noticed Caesar and Varla arrive but knows nothing about the mutants. As G and Isaac shoot their way through the prison they finally reach the electric chair theater where Varla and Caesar are strapped into the chairs. Clement then appears and, in a plot twist, reveals that he was behind the mutant outbreak and that Caesar was just an accomplice. Clement then executes Caesar, robbing Isaac of his revenge. Clement explains that his experiments were to save his dying mother (Following this, it is implied Clement has an incestuous relationship with his mother). Clement then descends on a giant elevator along with Varla, his mother, and Ceaser's corpse. Seconds later, Clement tells G and Isaac to look at the stage behind them were to gigantic, physically enhanced, convicts wearing black masks lumber in through the door. Clement reads the two convicts previous crimes which include murder, assault, rape, and even eating a little boys puppy. One of the convicts, Brutus, suddenly turns around and beats his fellow convict to death before turning around and firing an arrow at you from a crossbow attached to his arm. The fight starts inside what seems a generator room where the player scurries from place to place while shooting Brutus's weak spot (The crossbow attached to his severed arm)
After descending the elevator in pursuit of Clement in the final episode 'Overkill', G and Isaac shoot their way through hordes of mutants inside Clement’s underground lab before finally reaching his center of operations. They discover Varla’s brain has been taken out, kept alive in a jar, while Clement’s mother’s brain has been transplanted into Varla's body. At first it seems the experiment was a success, then Varla’s body begins to vomit and she falls into a pit after being machine gunned by G and Isaac, mutating her into a giant monster. G and Isaac draw their guns and aim them at the newly mutated mother. The duos' statements lead gamers to believe that they are about to play a balls to the wall, frantic last stand shootout against the giant mutant, but then the screen cuts to a missing reel screen for a few moments. Once the reel resumes G and Isaac are outside claiming victory over Clement and the giant monster, thanking the use of miniguns they found randomly lying around nearby. However, the monster returns and the pair battle her again. Afterwards, Clement appears and seeks to atone for all the problems he caused by climbing into his mother's womb. G and Issac are picked up by a helicopter along with Varla’s still active brain in a jar, Issac presses the button to explode the factility and G said Varla is the hottest girl he have ever met and as the compound explodes they realize the helicopter is being piloted by a mutant.
After the credits roll, Papa Caesar's tape recording from before is played, revealing a message to Isaac that Clement's plans were small minded but he had powerful friends, and that Isaac's father is still alive.*

Personally, I don't think this game fits on a Wii. To look at the age ratings this game has gotten, they're all 15+/18+. When I think about a Wii, I think about Wii Sports, Mario, Sonic, a little kid who breaks the TV. Not about a mutant shooter game. The storyline is fair. This is a regular shooter game, with mutants, and I don't see many specials of it. The price/quality is fair.