New Delhi, Aug. 25 -- The power station is small by any measure - one of hundreds of "peaker" plants scattered across Britain, kept in standby for demand surges that rarely last more than a few hours. When it went dark in July, nothing failed catastrophically. Britain's National Grid adjusted. Nobody's lights went out. What happened instead was something more durable: a facility whose operators had no reason to expect they were a target discovered that they were one, and it stayed offline for four days.

a private escalation that acknowledged the sensitivity of what had been found.

The four-day duration of the outage drew more attention from security professionals than the attack itself. Peaker plants are not normally complex to restore ...