If you're a fan of Lost, you already know who Martin Keam is. If you don't watch the show, you're missing out! First some background about Martin Keamy:
LOST SPOILERS AHEAD
Martin Keamy makes his debut in Season 4 of Lost.
He is a former U.S. Marine who has done mercenary work as a gun for hire in places such as Uganda. In Lost, he is working as a mercenary for Charles Widmore:
Widmore was originally from the island. He was/is a "Hostile" or "Other" (as they are more commonly known today). At one point, there was a power struggle between him and Ben Linus. Ben Linus allegedly "stole" the island and leadership over the Others by tricking Widmore to leave the island. Widmore has put together a group to travel to the island by freighter with the objective of capturing Benjamin Linus (aka Dean Moriarty and probably other aliases):
Widmore's freighter consists of two teams who are trained to infiltrate the island. A science team comprised of an anthropologist, a physicist, a "ghostbuster" (he talks to dead people apparently), and an alcoholic pilot. Then there's the mercenary team headed by Martin Keamy. There's also the ship's crew members who aren't really part of either island infiltration team.
Eventually, Keamy and his mercs have Ben and some others surrounded, and Keamy tries to "shock and awe" Ben into surrendering. When that doesn't work, Keamy tries a different tactics:
*WARNING MAJOR SPOILERS IN THIS CLIP*
http://www.youtube.com/watch?v=Rlg4qh6ohI4
Ben is eventually taken into custody, but he is freed by the Others with some help from Kate and Sayid. Keamy eventually tracks down Ben again, but this time Keamy has taken out some "life insurance" by rigging his body with a "dead man's trigger":
While a lot of the above mentioned events had been going on, others had been evacuating survivors of Flight Oceanic 815 to the freighter. Keamy, however, has rigged the freighter with enough C4 to blow it all to hell, and the trigger is a device on his arm that is connected to his heart. If his heart gives out, in other words, the C4 will detonate and kill everyone on the freighter.
I don't want to spoil too much more, so I will stop here, but that's the story of my PSN. :D