On May 21, 1946, Louis Slotin, a physicist working on the bomb development in a secret laboratory near Los Alamos, N.M., was demonstrating a procedure known as a crit test. The test was meant to verify that the plutonium core of an atomic bomb had the right size and heft -- the critical mass -- to sustain the chain reaction among atomic particles that would cause an explosion. Performing the test was risky; Richard Feynman, the Nobel Prize-winning physicist, referred to it as ''tickling the dragon's tail.''

But Slotin (pronounced SLOE-tin) had done it so many times, often pushing the limits of safety to attain better data. This time, however, his hand slipped at a crucial moment, the core ''went critical,'' and Slotin was zapped with a dose of radiation that killed him after nine days of increasing agony. The other men in the room were also exposed, but they survived, largely because Slotin absorbed most of the radiation. Some thought him a hero.

This is the story that inspired ''Louis Slotin Sonata,'' a play by Paul Mullin at the Ensemble Studio Theater, and as a historical episode suitable for dramatizing, you can't do much better. The lead production of First Light, the theater's laudable annual festival of science-themed plays, it has a singular and terrible event that is grotesquely compelling in its own right but has natural resonance beyond itself. And it has a sympathetic protagonist of intellect, wit and fatal hubris. The play is at its best when it is telling its central story: a good man doomed by his own hand, the tormenting experience of his final days and its emotional repercussions among those around him.

Sticking close to historical fact and incorporating medical reports and other documents into his text, Mr. Mullin has fashioned a crafty narrative propelled by the secondary characters: other scientists, doctors, a nurse and Slotin's father. They pass the story line around like instrumentalists sharing an orchestral theme, and as directed by David P. Moore, the pace is vivace. (Musical metaphor notwithstanding, the claim of the title and the playwright -- that the play's structure is based on classical sonata form -- is irrelevant and pretentious.)