Numerous ordinary storylines combine to offer the plot for this slightly above average episode. The main story is really more about encountering a new lifeform and our response to them w/o common ground. On this occasion the lifeform is very alien compared the normal humanoid form - Weser's nanites. Two of these modified microscopic organisms escape his experiment and evolve (tying in with the episode title) into lifeforms that can replicate themselves. Something of a precursor to SG-1's Replicators I guess.
The beginning of show is just basic plot setup. It involves a Dr. Stubbs who has long been preparing a for scientific experiment only doable every 190+ years or so. The nanites present the real only reall threat from the start by eating up the datacores of the main computer, commencing a series of mulfunctions that place the ship in jeopardy. This has the effect of threatening the execution of Dr. Paul Stubbs experiment. To prevent this Dr. Stubbs kills a huge cluster of the nanite population and commences the a much better middle and final acts as the crew and the Dr. are threatened with retaliation with threat to life.
Here begins a fairly (now) predictable series of scenes where the crew struggle to identify and then communicate with the alien lifeform. Will they do it and save the ship? Will they live in peaceful coexistence? Will the experiment be saved?
Well the start is slow, but the mission does pick up in the middle, but levels out during the finale. The ending is full of resonance, providing a satisfactory if anti-climatic ending as you probably expect if watching it now.
So the episode is basically a first contact scenario, with the twist that these lifeforms are created by Wesley who then has honesty and guilt issues to contend with. Dr. Stubbs then commits genocide akin to squashing a collective of ants and because neither race knows of the others existance, the plot focuses on communication issues, in which Data does a funny job of approximating a high pitched computer voice! :D
A good start to this season, but nothing spectacular!