Part 3 Chemistry summary:

Sn2 Reactions; What are they?

This part depicts an Sn2 reaction, a bimocular nucleophilic substitution reaction. Christine and Lauren are approached from the back by Lisa, who is a nucleophile. After the three of them practice together in a representation of the transition state where the central carbon atom (Christine) is bonded to both the nucleophile (Lisa) and leaving group (Lauren) at the same time, Lisa kicks Lauren out of the group because she's a better fit for the group. All of this happens in one step, with Lisa approaching and Lauren leaving all at the same time.

Extended Summary here.