Biotech stocks are plummeting below the previous February low. That's bad but not as scary as this thought: What if the only sentiment-turning catalyst left for biotech stocks this year is the outcome of  Eli Lilly's (LLY - Get Report) Alzheimer's phase III study expected around Christmas?

With the Brexit vote only adding to the risk-off, no-reason-to-buy sentiment prevalent in biotech today, it's going to take a medical miracle -- a drug or technology that's a true scientific game changer -- to convince investors biotech stocks are the place to park new money.

Lilly's solanezumab, the beta amyloid-targeting Alzheimer's drug, could be that game-changing drug. And so might Biogen's (BIIB - Get Report) Alzheimer's drug aducanumab, following close behind.

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The problem, of course, is... well... everything. Beta amyloid drugs have not worked against Alzheimer's disease, ever. Lilly has already run solanezumab through two, large phase III studies without showing statistically significant improvements in cognition and function of Alzheimer's patients. Biogen's aducanumab has looked promising in an early-stage study, but only in a small number of Alzheimer's patients, prompting skepticism.

Hope springs eternal in Alzheimer's drug development, however, so Lilly is taking a third shot with solanezumab. The ongoing phase III study limits enrollment to patients with mild Alzheimer's where beta amyloid drugs might show a greater benefit. Lilly has also narrowed the study's primary endpoint to cognitive improvement only (function is now a secondary endpoint) which might up the odds of success, at least from a headline standpoint.

Lilly has told investors to expect an announcement of the solanezumab trial results in December, near Christmas.

Investor sentiment surveys done by the sell-side over the past several months swing 60-70% predicting another solanezumab study failure. Pessimism is not surprising giving the dismal history of Alzheimer's drug development.

But let yourself daydream a moment about a scenario where solanezumab works, perhaps not overwhelmingly, but enough to get approved by lenient regulators cognizant that Alzheimer's patients have few good treatment options. Millions of people suffering from the memory-robbing effects of Alzheimer's would want to start taking solanezumab in hopes it would slow the course of the disease. For Lilly, that would translate into billions of dollars in sales.

Lilly is Big Pharma, so why would solanezumab's success excite investors enough to get back into biotech? Because Biogen's aducanumab also targets beta amyloid but is projected to be more potent based on the early clinical data. A big phase III study is already underway. A win for Lilly would be seen as a big win for Biogen.

Right now, Biogen trades as if its pipeline is worth nothing. If solanezumab works and investors believe in Alzheimer's drugs again, Biogen's stock price moves a lot higher. The entire biotech sector could be lifted with it, and everyone ends a difficult year on a high note.

And then we all woke up from that amazing dream. Reality is harsh. The Nasdaq Biotechnology Index closed Monday at 2524, a new nadir for the year, down 33%. In fact, the last time the NBI traded this low was May 2014. The iShares Nasdaq Biotechnology ETF (IBB - Get Report) , which tracks the NBI, is under-performing similarly.

Unfortunately, the biotech calendar of sentiment-shifting clinical catalysts for the rest of the year is looking a bit thin.

Bluebird Bio (BLUE - Get Report) will use the American Society of Hematology annual meeting in early December as the forum for an update on its gene therapy programs in sickle cell disease and beta thalassemia. Mixed data and a lot of unanswered questions raised last December put Bluebird in the investor penalty box. The company needs to erase doubts this time around.

In July, Sage Therapeutics (SAGE - Get Report) will announce results from a mid-stage study of SAGE-547 in postpartum depression. This study is a big deal for Sage but probably not game-changing enough to shift overall biotech sentiment.

Kite Pharma's (KITE) first re-engineered T cell-based cancer drug, the CAR-T known as KTE-C19, will have results from a phase II (but registration-quality) study at the end of the year. Data from this study will form the basis for Kite's first approval filing, which is widely expected. There's more downside risk to Kite than upside if something goes wrong.

Biotech stocks need a big assist from Lilly if the horrible performance in the first half of the year is going to turn around for the better.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.