The dreams of the future come to young Allison at very effective times, always paying off dramatically in her life soon after-wards. Each dream makes young Allison more and more nervous and scared. What's more important, it's only three quarters into the episode that we fully learn what kind of person future Steven is. Then comes future Steven's fantastic shock revelation of a very special day in young Allison's time. Young Allison awakens from the dream to find that something terrible has happened to her friend and that Steven is responsible. In the final act, young Allison faces one of the classic "what if..." questions, only this time she is actually in the position to choose the answer to that question. And the decision she makes gives us an enormous insight into secrets in present day's Allison's mind. The scene where she makes the decision is almost unbearably tense and the aftershocks are shown to us briefly but effectively. The coda in present day has a nice eerie vibe to it, releasing the tension but still leaving us thinking.
The actress playing teenage Allison and the actor playing teenage Steven are both very good, especially in the scene on the street where young Allison says they should not see each other. Her acting in the scene where young Allison makes the decision is also gripping. Hopefully they will both get opportunities to develop their talents.
In conclusion, a trick episode that turns out to be emotionally one of the most affecting episodes of Medium ever.