Lucas: "Hanna! Don't struggle! I am not here to kill you!"
Hanna: "Where is Caleb? You killed him and now you're going to kill me! I have completely lost the thread of what's going on!"
He chases her around her bedroom for a while, yelling things that aren't really very informative, and she dials Caleb's cell phone so that he can hear their conversation, which, again, is not very informative. What she doesn't know, though, is that Caleb is even now rushing up the stairs, so there will be three people yelling and fighting literally across the hall from Ashley's bath time, and she won't care, because she is drunk.
Lucas: "Hey, Caleb. Look, guys. I'm really sorry. The truth of the mystery of me, which began at the hotline and continued to the canoe and is still going on, is both very simple and oddly contrived. Come with me now to another show you don't get to see on this show, wherein I explore the secrets."
It turns out that Caleb left Lucas with a shit-ton of money -- ill-gotten gains from his various werewolfing activities -- when he went to California to visit his new family. Lucas, with his interest in sabermetrics and whatever, tried to invest this in what he believed was a sure thing, but then one of the baseball teams in his elaborate system got disqualified, throwing off his brackets and bringing the whole thing down around his ears.
This is what he was crying about on the hotline, and it's why he's been so weird about Caleb being home, and it's also what he was trying to tell Hanna in the boat: He wanted her to back him when he finally told Caleb, and was being so sketchy because he was terrified that one or both of them would break up with him. Sad little tale, to be one that we never even heard of until just now. Where he's been since then, without Hanna's support and still anguishing over hurting Caleb's feelings, has been touring the nearby towns selling off his comics and collectibles to pay Caleb back.
Caleb, immediately and awesomely: "We'll work it out."
Lucas: "Check. And Hanna? Will we also work it out?"
Hanna: "Hit me up tomorrow, because I have fucking had it with you today."
Fact: Obviously it was not Lucas that beat hell out of Emily at the greenhouse.
Fact: Obviously it was not Lucas that replaced Hanna's vodka with lake water.
Fact: Obviously Hanna jumped the gun when she tried to kill him on the rowboat.
Conclusion: Mona Vanderwaal is A, murdered Alison with her bare hands, and has psychic powers.