The scene in Chapter 2 where a drunk and crying Ally tells Lisa what happened to Alan's ex-girlfriend Marion. See the Nightmare Fuel page for the rest.
The really heartbreaking part of that scene was Ally flashing back to Marion using the safeword in desperation to try to get her to untie her bonds, which Ally couldn't do for reasons outlined in the Nightmare Fuel page. For a domme like Ally, being put in a position where one has to break the rule of the safeword to keep someone safe is one of the worst experiences that one can go through.
Ally and Lisa's cycle of denial that they are anything more than Just Friends and utter prideful/fearful refusal to just talk to each other about their feelings for each other comes to a head at the end of Chapter 4, when the two of them completely emotionally misinterpret things following the night with the ropes (one of the greatest sexual experiences of Lisa's life), and they take Anne, their newly-met friend, to the Crimson. Both of them are fully convinced that something is going on with the other and Anne (Lisa in particular being jealous and possessive), and Ally gets it into her head to try a threesome, which Anne is not on board for. She tells the both of them to go to hell and leaves, Lisa yells at Ally for treating their friend like this (proving herself a "weapons-grade hypocrite"), and Ally completely breaks Lisa's heart by telling her "fuck you and your friendship", leaving both of them devastated.