two women, nicky bones & valentine mercier, were aspiring spies. they trained together for two years, & they were partners on each assignment, val as the top trainee & nicky as the second best. val was always more talented at both espionage & combat than nicky, but nicky adored her rather than regarding her as competition. val, by nature manipulative, spent most of their initial training mistrusting nicky's motivations. she only slowly accepted nicky as sincere, but when she did, she put more of her trust in her than in anyone else in her life.

however, they were on their third mission as agents of a private organization when nicky was cornered by the u.s. government. she was taken hostage, & for six weeks, val didn't know if she was alive or dead. when she next heard word of nicky, it was when an agent informed val that a senator was found dead in his home, & someone with nicky's eyes & a scar across her throat was caught for a moment on camera near his estate.

the next two years were spent apart: val worked for the agency, & nicky murdered a variety of men. they each become renowned in certain circles for their respective abilities. both followed any news of the other eagerly, but never attempted direct contact. val was assigned a new partner, lina, older & more experienced but too similar to val for them to connect.

this pattern breaks when nicky is contacted by the u.s. government. she's presented with an ultimatum: "we know where your sister lives. kill agent valentine, or your sister dies." nicky reluctantly sets out to find val to assess her options. when nicky finds her, she halfheartedly attacks; val retaliates violently. the conflict is drawn-out, nicky buying time & val trying to complete her mission while dealing with nicky. the longer they spend fighting, the more nicky becomes certain that she can't kill val, because val is both still a better agent than nicky, & because nicky still loves her.

after several clashes, nicky slips up. val finally figures out who exactly has been pursuing her, & loses herself in feelings of betrayal & rage. in the midst of this, lina also learns who nicky really is, & confronts val about it. they argue, but ultimately lina convinces val that the only route forward is killing nicky.

as their fights continue, val becomes more desperate to kill nicky, but nicky's own efforts become more & more token; she doesn't actually want to hurt val. inevitably, val gets the drop on nicky, & she has her gun to nicky's head. & she hesitates. she says, "do you really want me dead?" & nicky says "of course not. i'd rather you kill me than live in a world without you." & nicky signals to the CIA bug planted on her. val, finally understanding, shoots the bug. briefly & tenderly, they kiss.

nicky & val evide lina, & go to rescue nicky's sister, a woman named harper bones. lina shows up at harper's apartment to turn them in, but harper is about to be killed by a CIA sniper & lina takes the bullet for her, knowing that harper is uninvolved & innocent. val decides to take lina with them while she recovers from her gunshot wound. the four women leave the country, lina reluctantly taking harper under her wing.

half a year later finds all four still living together. in a tiny ceremony not recognized by law, nicky & valentine exchange rings, with the vow that nothing will break them apart again.