- "You have seen things that no other man alive has seen, been given messages intended for the Order that must never fall into the hands of our enemies. Don't be so selfish."
- ―Sergei to Nikolai after he refused to return to Russia, 1926.[src]
In 1911, Sergei recruited a woman into the Brotherhood, who later became his wife. In December 1917, they joined a community of scientists, revolutionaries, and Assassins at a facility in Protvino provided to the Brotherhood by the Russian Academy of Sciences.
In late 1918, he met with Nikolai Orelov in Moscow, who had requested the Assassins to protect Princess Anastasia Nikolaevna from the Templars. Informing Nikolai that he had to report to the local Assassin bureau, Sergei promised to bring Anastasia to meet their Mentor, but she instead ended up in captivity inside their hideout near Spasskaya Tower.
- "Under orders from our hidden leader, Sergei left our compound to hunt down Nikolai Orelov, the defector, and reclaim the Shard of Eden in his possession. It was a move born of desperation, and we have not heard from him in two years. I fear the worst."
- ―Sergei's wife, 1928[src]
In 1926, Sergei was tasked by the Mentor – hiding from persecution by the Templar-influenced Stalinist regime – to track down Nikolai Orelov, who had left the Brotherhood and fled Russia with his family, taking the shard of the Staff of Eden that was destroyed in the Tunguska explosion with him. Sergei tracked Nikolai down to Connecticut in the United States in January 1926, with help from Assassins within the Bureau of Investigation.
Arriving at Nikolai's cabin in the woods, Sergei found his son Innokenti and asked where his father was. Hearing voices, Nikolai walked out of the cabin, and Sergei urged Nikolai to come back to Russia with him. Upon hearing Nikolai's refusal, Sergei expressed his disappointment at Nikolai's decision to keep the information from his vision at Tunguska to himself, stating that it must never fall into the hands of the Templars.
Sergei then grabbed Innokenti, calling Nikolai greedy for having taken the shard of the Staff with him, and wondered if the Assassins should not take something small in return, holding a knife up to Innokenti's face. He was taken off-guard when Nikolai disarmed him with a frying pan, and was knocked to the ground. He was then strangled to death by Nikolai, whilst futily attempting to stop his former comrade.