Beneath by Ariel Sullivan

Here is a summary with spoilers for Beneath by Ariel Sullivan

Six years after a nuclear war devastates the surface of the Earth, the remaining survivors live beneath the ground in Haven, a settlement divided into sectors that determine each person's occupation and status through color coded identification bands and health scores. Sasha Cadell works as a medic in the Ward, where she became known as "Death's Angel" because she was immune to the illness spreading through Haven and stayed with dying patients so they would not die alone. She likes being in the ward, her last name has bad image with her dad during the previous war, so she almost uses the ward as a hiding place. That is until a man is brought in; Tristan Hayes, and Sasha stays next to him as he heals. The two become friends over time, and Tristan is able to find out that Sasha is actually quite good at combat. Her father secretly trained her during the war, but she doesn’t want anything to do with it. Sasha spends her time with her almost boyfriend Jaxon, and haning out with Tristan every now and then, despite him being a Commander. Sasha is also haunted by the deaths of her family, especially her younger brother who she watched die right in front of her during the war. 

Two years go by like this, and Sasha gets called to command. She is given the command to leave the Ward and transfers to Expansion. More specifically, Commander Tristian Hayes of the elite Force Unit Seven recruits Sasha to become the team's medic after remembering how she secretly saved his life years earlier and continued treating his team despite Haven's strict regulations. Sasha reluctantly joins the unit, where she faces hostility from Ingrid, who blames her for the death of her former teammate Lily (which is the reason they now need a new teammate). The Unit is very suspicious of her at first because they know she doesn’t want to be there, so why should they trust someone who deosn’t want to be there. Sasha trains with Unit Seven, forms friendships with Levi, Damien, Patrick, Isla, and Rumi, and begins to earn the group's trust despite her emotional distance and guilt over her past.

Sasha’s first training session she realizes how unprepared she is when she has a moment of PTSD. During exercises inside a holographic simulation known as the House, she relives the murder of her younger brother Eli and freezes during the scenario; Tristan offers a bit of comfort during this time. Other members of the Force begin targeting Sasha after Commander Burdon publicly encourages attacks against her, but Unit Seven repeatedly protects her. Levi becomes Sasha's training partner and pushes her to improve her endurance, confidence, and combat skills. After her breakdown at training, and a rough training session with Levi, Tristan comes for her and tries to kiss her. Sasha pushes him away. 

Tristian eventually reveals the true purpose of Unit Seven's missions. Haven has less than a year of supplies remaining, and the exploratory teams have been searching for a hidden cache of resources somewhere within the unmapped tunnels known as the Abyss. The cache is believed to contain enough supplies to save Haven, but every previous expedition has failed because of deadly traps guarding the location. Sasha agrees to fully commit herself to the mission while her relationship with Tristian slowly develops into a romantic one as they talk about the almost kiss that happened.

Unit Seven travels to the surface (the above) and crosses the radioactive wasteland before entering the Abyss. During the search, Sasha and Levi trigger one of the explosive traps protecting the hidden cache. Levi suffers severe injuries, including heavy bleeding and a damaged liver. Sasha performs emergency treatment, gives Levi her own blood through a field transfusion (another reason she is valuable to the team, she has the blood type that she can donate to anyone), and drags him back to safety. Despite getting him back, Sasha feels immense guilt, and a little anger towards Tristain because she was preoccupied making sure he was safe as well rather than looking after her own partner.  The mission does give Sasha her first official call sign - The Beast. After the team returns to Haven, Burdon assigns Jaxon to Unit Seven in an attempt to undermine Tristian's leadership, and since Levi can’t return to the field yet. Tristan is furious at this choice, especially since Jaxon immidelty tries to go for Sasha; the team also doesn’t trust him. Sasha secretly exchanges her medical records with Levi's, transferring his poor health score onto herself so he can remain in the Force. Dr. Kumar discovers the deception but allows it to continue. 

Meanwhile, a deadly illness spreads throughout Haven, overwhelming the Ward. Ingrid asks Sasha to save her younger sister, which would be going against command. Despite not being in the ward, Sasha wants to help out, so she is working overtime. Tristan comes to visit her and Sasha says she just wants a distraction. He kisses her and does a bit more; but then ends up leaving. Which makes Sasha feel very sad and weird that things didn’t go further. She goes to Levi to ask more about Tristans background, which is how she found out about a betrayal. She now feels really bad and decides to go to his room and talk to him one on one. They end up kissing and sleeping together that night. 

This illness goes on for weeks, and it starts spreading, and it even gets to Dr. Kumar, Sasha’s old Doctor she reported to. Kumar reveals that he secretly watched over Sasha because her late sister Lara had asked him to protect her. Before dying, Kumar gives Sasha a small metal chip and instructs her to "keep hope alive," "find the snake," and "protect the dead." 

Sasha and Tristian finally acknowledge their feelings for one another and begin a relationship, although Sasha still struggles to tell him she loves him because she fears losing everyone she cares about.

Realizing that Burdon can no longer be trusted, Tristian, Levi, and Rumi organize a mutiny. Unit Seven escapes Haven and returns to the Abyss with Kumar's chip. Sasha inserts the chip into a concealed mechanism, activating ancient technology that opens a massive hidden storage facility. Inside are enormous supplies of medicine, equipment, construction materials, and other resources capable of saving Haven (and other random important humanity things like a Picaso painting). While exploring the facility, Sasha discovers a room containing pods labeled Tactical Artificial Biotech Intelligence Tracking Human Algorithm, abbreviated as TABITHA. When the chip activates one of the pods, a woman briefly appears inside before disappearing. Sasha tells only Tristian about the discovery, and he decides to keep it secret. Kaleo then arrives and reveals that he, Rumi, and Kumar had secretly worked together for years against Burdon. Rumi also reveals that she and Kaleo have been in a secret relationship for two years. Kaleo explains that the hidden tunnel network has been maintained by the Moles, members of Expansion who fake their deaths to secretly work outside Haven's leadership.

When the group returns to Haven, Burdon has already uncovered Sasha's falsified medical records with Levi and learns enough about the mission to stop the rebellion. She forces Tristian to become her second in command, strips Unit Seven of its exploratory duties, and assigns the team to guard duty. Sasha's altered health score permanently disqualifies her from serving in the Force, and she is exiled back to Expansion with a gray identification band. During the confrontation, Sasha realizes Kumar's warning to "find the snake" referred to Henderson, commander of Unit Twelve and Damien's boyfriend, whose call sign is Snake. Henderson unknowingly became Burdon's source because Damien trusted him with Unit Seven's plans, allowing Burdon to learn about the rebellion. Rumi silently confirms that she has reached the same conclusion. Kaleo is also removed from his position and sent to Expansion. The novel ends with Sasha returning to tunnel excavation, but she no longer intends to simply survive. She resolves to continue fighting against Burdon's control, protect the people she loves, and help rebuild Haven. The hidden supply cache has been secured, the mystery surrounding TABITHA remains unresolved, and the story closes with Unit Seven separated but still committed to changing Haven's future.