The Enterprise crew rescues the occupants of a disabled ship in a decaying orbit and Picard subsequently finds himself in the middle of a dispute between the representatives from two societies — the Brekka and the Ornara — who are involved in a business transaction regarding some valuable cargo. The Brekka's payment for the cargo went down with the ship, so the Ornara refuse delivery, and we have a problem.
The problem becomes a moral quagmire when the cargo is revealed to be medicine desperately needed by the Brekka, and the situation is further compounded when Crusher determines the medicine is actually an addictive narcotic the Brekka don't actually need in order to survive. The Ornara benefit greatly from the Brekka's dependency on the drug, which has permitted the Ornarans to advance their society while the Brekka have been treading water for the past 200 years. Crusher desperately wants to free the Brekka of their drug addiction, but Picard notes that this would be a blatant violation of the Prime Directive.
The Prime Directive can make for an interesting debate, and it's nice to see Crusher's distaste over the situation even as Picard defends it as a necessary tenet. But again, a key problem with "Symbiosis" is that it oversimplifies the story to a point that we're forced to wonder how, after 200 years, an entire society can uniformly be addicted to a drug with no knowledge that they're being exploited by their "symbiotic" partners in drug-dealing/addiction. They're hopelessly incompetent ship-runners, which makes you wonder how they even survive.
Simply put, "Symbiosis" — even though it tries to be about something real — is ultimately too heavy-handed and simplistic to work. There's a point in the story where one Ornaran actually makes an evil grin when Picard confronts her with the fact that he's on to their exploitative behavior. This betrays the story as unintended parody more than parable.