Plot Summary (Spoilers), please fill any gaps
Okay, K. Turner is Mary Ann, a girl from the midwest. Her best friend is Matty. They both move to Chicago, where Mary Ann gets into drugs and other problems (probably prostitution, maybe murder). She is helped out by a lawyer, and works in an office for awhile.
She later marries someone (maybe the lawyer), who tries to do a deal with Mary Ann's 2nd husband (Crenna), but fails. Crenna steals her away. However, before this point, she has assumed Matty's identity, proably to hide her past.
She is forced to sign a pre-nup by (smart) Crenna, and then starts thinking about how to kill him. She starts looking for a lawyer patsy about a year before the time the movie occurs, and meets the one lawyer who tells her about Ned. She then schemes to meet him by attending a concert she somehow knows he'll be at.
She seduces him, making him think it's his idea. Gets him sexually addicted, and then pursuades him to kill the husband, again making him think it's his idea. Before the killing, she changes the will, forging Ned's signature, and deliberately messing it up so that the husband will die intestate, and she'll get everything.
She always plans to stick Ned with the crime, so she could keep all the money. She also kills the real Matty at some point, perhaps to cut her out of the money as well. She presumably sends the cops the glasses with Ned's prints to implicate him. She also hires the arson guy to rig the boathouse so Ned will die. But Ned finds out that she was looking for a Patsy a year before, grabs a gun, and forces her to open the boathouse.
The only real Q here -- Matty is already dead in the boathouse before it explodes. Mary Ann is presumably planning on Ned setting the bomb and killing himself, such that his body and Mattys are both found there. But how did Matty manage to trigger the bomb without hurting herself? Must have had another trigger, like Rourke was discussing. Other Q -- did she know Ned would make her try the door? Finally -- why did Ned change his mind at the end? Maybe he thought the "hot looker" Rourke referenced was actually the real Matty, not Mary Ann (Turner).
But bottom line -- Mary Ann (Turner) was clearly very cold-blooded. She was willing to dump the first husband, kill the second husband, kill Ned, and kill her best friend from childhood, all to get the money.