You are an ex-Kell Hound member, a fairly raw recruit that was just inducted into the Hounds when the Battle of Mallory's World happened. In the aftermath, Morgan Kell semi-disbands the Kell Hounds, reducing it from regimental size to battalion size and then goes into retreat at Zaniah. You were one of the 'mechwarriors let go. But Morgan tells you to be ready to return to the Hounds because there will come a day when the Kell Hound Regiment WILL be reformed. You get to choose your 'mech here (a standard light 'mech, 3025 TRO vintage).
While on Galatea, looking for work, you meet a guy who has use for a 'MechWarrior who is fairly unknown but still has good references (which you fulfil because a) you were a Kell Hound, no matter how brief, and b) it was very brief). He claims to be a "connector", someone who introduces people to other people who has use for them. Since you are running out of money for rental (both for yourself and the 'mech), you agree to meet his contact. This guy introduces you to his client, a woman who reveals that both of them are actually part of a covert Lyran team (yeah, he was lying) trying to find out information on a series of attacks by extremely well equipped bandits in the Periphery. Several Lyran worlds have been hit and the local garrisons overwhelmed by sheer force. They are looking for additional muscle, so to speak, and you fit the bill, especially since you came with your own 'mech.
As a sign of how important the Lyran state view the threat (although you are never given concrete proof that they are working for the Lyran state) your team is assigned a small JumpShip as well as a Leopard class DropShip. And so you embark on a series of missions over several worlds and the mystery deepens. You encounter the pirate 'mechs on a Lyran world and they are powerful units. Not only are some of them Star League era 'mechs, those 'mechs have all been extensively modified (including a Crab with 2 PPCs and a large laser; and another Crab with 3 large lasers, jump jets and greater speed). Eventually, you find out the location of the pirate base and try to attack it but the defenders have overwhelming firepower. And I do mean overwhelming. However, you get the chance to destroy the first wave coming at you because you do have the element of surprise, but you can see the second wave is going to stomp you flat. It is either run away or get your 'mechs wrecked. With no other choice, you decide to infiltrate the base (which is actually in the ruins of an old city that has been long abandoned) on foot with the lady. The man stays behind with the 'mechs as backup. Hijinks ensues. Pirates get killed, sex slaves gets rescued, female gets wounded, male saves the day in a 'mech, although you get to be the big damned hero rescuing the slaves.
Wounded female gets evacuated back to the Commonwealth on board the Leopard, and you are stuck on the Lyran planet until another DropShip arrives to pick you up and continue the search. One of the former slaves turns out to be a 'mech pilot. Not military, but civilian, but still a 'mech pilot. You get to pick up another 'MechWarrior who is a survivor of the local garrison that was massacred before you arrived. Information from the former slaves indicates that the pirate base you hit was only a forward base. It is not their main base, which means they are far bigger than anyone realise.
This is just the opening chapter. You will eventually find out why the pirates are so well equipped and what they are up to and what is the real story behind the Lyran operatives. Suffice to say that what you have been told are a blend of truth and lies and as the story moves forward, things will be explained, you will find Star League era stuff and a whole host of things that people of 3025 would never even dream about. The whole Inner Sphere will never be the same again. Well, maybe a small part of it :D