LONDON — In November 2006, Marina Litvinenko watched her husband, Alexander, die in a London hospital. The Russian spy -turned-whistle-blower — whom she called Sasha — had drunk green tea laced with the radioactive element polonium. President Vladimir V. Putin probably approved the murder, a public inquiry in Britain later said.

Last Friday, Ms. Litvinenko went to London’s Old Vic Theater to watch “A Very Expensive Poison,” a new play by Lucy Prebble, running through Oct. 5. She knew she would have to watch her husband die again.

In a telephone interview the morning after the performance, Ms. Litvinenko recalled how, as she sat in the theater beforehand, a woman walked up to her, shook her hand and said, “I think you’re very brave.”

“I didn’t know for which reason,” Ms. Litvinenko said — “for coming and watching the play, or fighting for Sasha.”