It was August 2020, and Yulia Navalnaya, the wife of Russia’s most famous opposition leader, was striding through the battered, gloomy hallways of a provincial Russian hospital, looking for the room where her husband lay in a coma.

Aleksei A. Navalny had collapsed after being given what German medical investigators would later declare was a near-fatal dose of the nerve agent Novichok, and his wife, blocked by menacing policemen from moving around the hospital, turned to a cellphone camera held by one of his aides.

“We demand the immediate release of Aleksei, because right now in this hospital there are more police and government agents than doctors,” she said calmly in a riveting moment later included in an Oscar-winning documentary, “Navalny.”

There was another such moment on Monday, when under even more tragic circumstances, Ms. Navalnaya faced a camera three days after the Russian government announced that her husband had died in a brutal Arctic maximum-security penal colony. His widow blamed President Vladimir V. Putin for the death and announced that she was taking up her husband’s cause, calling on Russians to join her.

“In killing Aleksei, Putin killed half of me, half of my heart and half of my soul,” Ms. Navalnaya said in a short, prerecorded speech posted on social media. “But I have another half left — and it is telling me I have no right to give up.”