Sergei is initially obstructive and hostile to the Americans due to the Soviets' belief that Apollo-Soyuz is a ploy to learn Soviet military secrets, and he refuses to share even the most basic information with Margo; however, after Margo speaks to him again, he reveals the Soviets' suspicions to her, and she arranges a secret meeting at 11:59.
He listens to Margo play piano and compliments her on the "lively" performance. He reveals he's on a long leash with the KGB because they know he would never defect, so it was easy to escape his minder and make it to 11:59. He then asks whether the club is a secret place for her, which she confirms, and when Margo tells him the club was named in reference to the Doomsday Clock, he tells her about the Soviets' fears of nuclear war.
Inspired by the clubs' coasters, Sergei suddenly comes up with the idea for an androgynous docking system, and the two sketch out the initial designs before returning to JSC where Sergei (now rather drunk) finds Aleida Rosales in the docking model. He compliments her on her dedication, and she corrects a flaw in Sergei and Margo's design. The three work through the night to finish the design so that Margo and Aleida can present it the next day, and when the Soviets accept it, Sergei reveals the Soyuz frequencies. Margo does the same for Apollo.
The destruction of KAL 7 worsens tensions between the USA and the Soviet Union, and with the other Soviets, Sergei is kept under guard in his hotel room. Margo visits him with borscht to ask for help contacting Danielle Poole and Nate Morrison in Star City. He tells her he has no way of contacting anyone with the phone dead, so Margo promises to arrange a phone call and he agrees to do what he can to get them in touch with their astronauts.
Sergei is recalled to Moscow while it's decided whether Apollo-Soyuz will be continued. Before he goes, Margo visits him in his temporary office at JSC and manages to communicate the flaw with the Buran shuttle's O-ring, which could kill everyone onboard if the shuttle launched in cold weather (common in Russia). They express their hope of seeing each other again soon and become flustered. This is overheard by a KGB agent.
Sergei soon returns to Houston to continue working on Apollo-Soyuz. He compliments Margo on her appearance before revealing he has more directives from Moscow. They climb into the docking model to work out the details of the handshake. Sergei flips a coin to decide who will be the host, but they lose it and his pants get caught. Their faces are brought close together, only for Emma Jorgens to interrupt them.
Amid considerable tension between their countries, Sergei videophones Margo to announce the launch of Soyuz. Margo has become upset with him due to the launch of the armed Buran, and she coldly refers to him as "Mr. Nikulov." The Soviets continuously delay the docking between the two ships, however. The Americans become annoyed, and Sergei apologetically communicates the state vector for their next docking opportunity. This is in fact the number for a payphone outside of Star City's headquarters, and Sergei hurriedly answers it. He explains there is a mechanical difficulty on Soyuz, which Margo doubts. He asks her to trust him, but the last time she trusted him, the Soviets sent an armed Buran to the Moon. He retorts that Pathfinder is also armed but says he is like her, on the outside, trying to do the best he can. But Margo can only hold off for one more orbit before calling Apollo home.
The Apollo-Soyuz mission is completed, and Ronald Reagan and Yuri Andropov meet to diffuse tensions between their countries. Sergei calls Margo after the funeral of Gordo and Tracy Stevens to pay his respects, and he invites her to get a drink with him at the ICSE conference in the UK that summer. Margo does not know whether she can make it, though. When they hang up, it is revealed that Sergei was forced to make the call by the KGB. Their leader states that Margo opened the door with Buran and by the time she realizes she's working for them it will be too late.