"Liaisons" is in the spirit of TOS-era alien encounters, where aliens behave strangely, our human characters do not understand, and then the story's lesson is revealed in a long soliloquy at the end that explains everything that came before. (For some reason that seems like a TOS template, anyway.) The problems are (1) I don't really buy the rationale after all is revealed, and (2) much of what comes before that is either forgettable or tedious — when it's not being laughable, of course.
The story doesn't show all its cards until the end, but for the sake of simplicity I will explain it up front — the Enterprise is being visited by two ambassadors who have been assigned to learn about concepts that do not exist in their society and they don't understand. To that end, they've paired up with Troi and Worf to learn from them the concepts of, respectively, pleasure (via foodstuffs, not sex, mind you) and antagonism. (I suppose they already know enough about both antagonism and Worf to make the assumption that he's the right candidate, which makes the whole experiment seem unnecessary, but whatever.) A third ambassador (played by the always reliable Eric Pierpoint of TNG's TV contemporary Alien Nation) takes Picard back to his world in a shuttle, which instead crashes on a desolate planet, thereby giving the writers of the then-forthcoming Voyager ideas for years to come.
On the planet surface, Picard is rescued by a woman named Anna (Barbara Williams), who also crashed there and has been stranded for seven years. Picard assures them they will figure out a way to escape the planet using technology from the shuttle to call for help, but Anna is a little ... off. The two of them sit and converse, Anna expresses her gratitude, and it slowly turns into a riff on Misery. She won't let him leave, comes up with excuses for why not, becomes infatuated with him, and then "accidentally" destroys the shuttle transceiver she's supposed to be retrieving. Serious question: Am I supposed to laugh when kooky Anna finally throws Picard to the ground and repeatedly demands "Love me!"? Because I did.
Aboard the Enterprise the two ambassadors drive Troi and Worf crazy, but mostly Worf (who gets the most entertaining iteration of this particular storyline), who has to deal with a guy who provokes him at every turn — as if he were doing it on purpose! (Because, plot twist: He is! But for a reason! Whoa!) This finally results in Worf kicking the guy's ass (after, yes, threatening him with disembowelment) when he cheats during a poker game. Somewhat funny: yes. Relevant storytelling: hardly.
Turns out "Anna" is actually the ambassador in disguise, who attempted to get Picard to fall in love with her/him in order to understand the concept of love. Their society first learned of the curious concept from the logs of two people who fell in love after being stranded in a similar crash/survival situation — so he decided he'd replicate the scenario to see if he could make Picard fall in love with him/her. I'd say his experiment's biggest flaw was that he expected it to succeed on the timeline of a typical TNG romance, which is to say, immediately.
So what we have here is story that obeys the Trekkian mantra of seeking out new cultures and trying to understand them (in this case, from the aliens' point of view), but does so in probably the most prosaic and forgettable ways possible.